Multiplex image reproducing apparatus

ABSTRACT

A method of reproducing multiplex images wherein an electrostatic image is formed on an image retainer by using a common electrostatic image forming device, the electrostatic image formed on the image retainer is developed, the steps of the above are repeated to superpose a plurality of toner images on the image retainer, and the toner images is transferred on a recording paper by one step. The developings other than the first time developing are carried out in such a manner that the surface of a developer layer on a developer feeding carrier does not contact with the surface of the toner image on the image retainer.

.[.This is a division of Ser. No. 656,582 Filed: Oct. 1, 1984, now U.S.Pat. No. 4,599,285..]. .Iadd.This application is a continuation ofapplication Ser. No. 07/697,540, filed May 2, 1991, now abandoned, whichis a continuation of application Ser. No. 07/380,299, filed Jul. 12,1989, now abandoned, which is a Reissue application of U.S. Pat. No.4,679,929 issued Jul. 14, 1987, which was a division of Ser. No.656,582, filed Oct. 1, 1984 and issued as U.S. Pat. No. 4,599,285, andclaims the priority of Japanese 183152/83 filed Oct. 3, 1983, 184381/83filed Oct. 4, 1983, 187000/83 filed Oct. 7, 1983, 187001/83 filed Oct.7, 1983, 238295/83 filed Dec. 17, 1983, 238296/83 filed Dec. 17, 1983,and 13167/84 filed Jan. 26, 1984, as claimed in U.S. application868,020, filed May 29, 1986, now issued as U.S. Pat. No.4,679,929..Iaddend.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image reproducing method and, moreparticularly, to a multiplex image reproducing method of superposingtoner images on an image retainer having a photoconductivephotosensitive member by repeatedly retaining and developingelectrostatic images on the image retainer.

2. Description of the Prior Art

As the above-specified image reproducing method, there are known in theart the methods which are disclosed in Japanese Patent Laid-Open Nos.144452/81, 116553/83 and 116554/83.

In any of these methods, the image retainer has on its surface a layerof a photoconductive photosensitive material such as Se, and thedevelopment resorts to a reversal method in which there is applied to anelectrostatic image having a lower potential than that of the backgrounda toner for frictionally charging it with the same polarity. Accordingto this method, there arises a problem that the photoconductivephotosensitive surface layer is generally and relatively liable to haveits electrostatic image retaining performance changed by the chargingstep and to be subjected to the "toner filming" or have itsphotosensitivity degraded. As compared with such a positive developingmethods as in the ordinary electrophotographic reproducing machine,i.e., the developing method in which the electrostatic image has ahigher potential than the background so that a toner charged with anopposite polarity is applied to that electrostatic image, moreover, thedevelopment by the reversal developing method has a problem that controlof the toner application is so difficult that a sufficient developmentdensity cannot be attained or that a reproducing apparatus is liable tohave its inside blotted by the toner scattered.

As the method in which an image retainer having a dielectric surfacelayer is used to retain an electrostatic image on the dielectric surfacelayer, on the other hand, there is known in the art a method using anelectrostatic recording head, a method using a screen photosensitivemember (as is disclosed in Japanese Patent Publication No. 34616/79) ora method using a screen control electrode (as in disclosed in JapanesePatent Laid-Open No. 137363/81). The electrostatic image retainingmethods thus disclosed are superior in that the electrostatic imageretainability and the toner image retainability required of thephotosensitive member are separated of each other. In these methods,more specifically, it is deemed that the share of retaining theelectrostatic image is borne by the electrostatic recording head, thescreen photosensitive member or the screen control electrode, whereasthe share of retaining the toner image is borne by the dielectricsurface layer. Those methods are featured by that color data areretained consecutively and independently as the electrostatic image onthe dielectric surface layer. However, the toner image formed on thedielectric layer cannot be other than a monochromatic one at all times.

This is because the developing method is conducted by the contactdevelopment so that a previous toner image is disturbed or color mixingoccurs upon the development even if another electrostatic image could berecorded on the previously formed toner image.

As the method in which an image retainer having a magnetic surface layeris used to form a magnetic image on the magnetic layer, on the otherhand, there is known in the art methods, which are disclosed in JapanesePatent Laid-Open Nos. 90342/75, 100732/76 and 106253/81. Thesereproducing methods resorting to that magnetic image is excellent inthat the retainability of an electrostatic image by the corona dischargeor the like and the retainability of a toner image required of thephotosensitive member can be separated from each other. In thereproducing methods using the magnetic image, more specifically, it isdeemed that retention of the magnetic image makes whole use of theinside of the magnetic layer while not having its surface state changedas is different from the photosensitive member, and that the share ofthe toner image retainability is borne by the surface of the magneticlayer. Those methods are featured by that the color data can be newlyretained as the magnetic image independently of the toner image formedon the magnetic surface layer. However, the toner image retained on themagnetic layer cannot be other than a monochromatic one at all times.

The method disclosed in Japanese Patent Laid-Open No. 144452/81 retainsa color image on an image retainer: by forming an electrostatic image onthe surface of an image retainer, which has been charged by a charger,by first exposure means and developing it by first developing means; byforming an electrostatic image on the same charged surface by secondexposure means and developing it by second developing means; and byforming an electrostatic image on the same charged surface by thirdexposure means and developing it by third developing means. The methodthus specified has problems that the separate exposure means arerequired for the respective ones of the repeated formations of theelectrostatic images to enlarge the size of the reproducing apparatusand to raise the cost of the same and that synchronizations of theexposure of the respective exposure means with the image retainer haverelationships with the respective positions of the exposure means sothat the synchronous control is troublesome to make it liable to invitecolor shift. Moreover, each of the development of that method isconducted by the forced method in which the electrostatic image having alower potential at its exposed portion than that of the background hassuch toner applied thereto as is charged with the same polarity. In thatforced developing method, the toner for effecting the charge at the samepolarity as that of the charge of the image retainer is so used in thedeveloper that it may not be applied to the background. As a result, thereversal developing method has a problem that although the toner isrepulsed by the background potential so that it is reluctant to inviteany fog, it is also reluctant to be applied to the electrostatic imagesso that a sufficient developed density can hardly be obtained.

Since the reversed image is obtained, according to this reversaldeveloping method, color reproduction of a positive image cannot beeffected so that the coloring is limited to the technique using the dotexposure of a printer or the like. In case it is intended to obtain apositive image when an ordinary original is to be reproduced, it isreversed, and the counter-measure for this reversal is difficult. Sincethe potential at the photosensitive layer of the exposed portion is atthe same polarity as that of the developer although it is low, moreover,the reversal developing method has a problem that the developer isreluctant to be applied to the electrostatic images so that it is liableto be scattered to blot the inside of the reproducing apparatus.

On the other hand, the disclosed in Japanese Patent Laid-Open No.144452/81 is one conducted under a non-contact jumping developingcondition in which the second and later developments by the reversaldeveloping method are conducted such that the layers of the developersformed by the developing means are not in contact with the surface ofthe image retainer. This method has problems that the development isreluctant to have a sufficient density and is liable to be blotted withthe toner scattered unless a strong bias voltage is applied to thedeveloping means to strongly apply the toner to the electrostaticimages, and that, the strong bias voltage is applied to the developingmeans, it is liable to leak to the image retainer or the like, or tonerof another color is liable to stick to the toner image developed beforeor the background.

The methods disclosed in Japanese Patent Laid-Open Nos. 116553/83 and116554/83 are substantially the same as that in Japanese PatentLaid-Open No. 144452/81 in that the formations and developments of theelectrostatic images are conducted by different means for the respectiverepetitions. As a result, those methods also have problems that thereproducing apparatus has its size enlarged to raise the cost, and thatthe synchronous control of the exposures of the respectie exposing meansis so difficult as to invite the color shift. Here, the method disclosedin Japanese Patent Laid-Open No. 116554/83 is different from the methoddisclosed in Japanese Patent Laid-Open No. 144452 in that the respectivedevelopments by the reversal developing method are conducted under thecontact developing condition, in which the developer layers formed bythe developing means brush the surface of the image retainer, thereby tosolve the problems of the reversal developing method that the sufficientdeveloping density can hardly be obtained and that the toner is liableto be scattered. Moreover, the method disclosed in Japanese PatentLaid-Open No. 116553/83 is different from the same Japanese PatentLaid-Open No. 144452/81 in that, in the second and later retentions ofthe electrostatic images, too, the surface of the image retainer isrecharged before exposure by the chargers, which are placed in front ofthe respective exposing means, so that toner in another color may not beapplied during a later development to the portions having the toneradhered thereto after the previous development. Since the second andlatter developments are conducted under the contact developingcondition, however, those methods have a serious problem that the toneradhered after the previous development is liable to be shifted duringthe subsequent development or to be mixed into the developer of thesubsequent developing means.

A prototype in which an electrostatic latent image is expressed in amulti-color image is concerned with a color image using anelectrophotographic system. This system of the prior art separates thecolors of an original through an optical filter and repeats thecharging, exposing, developing and transferring steps by using theseparated colors. In order that respective images of color particlessuch as yellow, magenta, cyan and black colors may be retained, morespecifically, those steps are repeated four times by that system. Therealso exists the so-called "dichromatic developing method", in whichelectrostatic latent images of different polarities are formed on acommon photosensitive member (or an image carrier) and are developed byparticles of black and red colors. These multi-color image retainingmethods are desirable, because they can add color data as compared withthe data obtained from the dichromatic images, but have the followingproblems:

(1) Transfer to a transfer member is required at each development ofeach color to enlarge the size of the machine and to elongate the timeperiod necessary for the image retention; and

(2) It is necessary to ensure the accuracy of positional shiftsresulting from the repetitions.

In view of these problems, there has been conducted a trial in which aplurality of toner images are developed in a superposed manner on acommon photosensitive member so that the transfer step may be finishedby one time to reduce the size of the machine.

As the developer to be used in this machine, on the other hand, thereexists a two-component developer, which is composed of a toner and acarrier, and a one-component developer which is composed only of atoner. The one-component developer has some problems in the chargecontrol of the toner but has advantages that no consideration isnecessary into the concentration and agitation of the toner and thecarrier, and that the size of the machine can be reduced.

The two-component developer requires control of the ratio of the tonerto the carrier but has an advantage that it is easy to control thefrictional charges of the toner particles. Since a magnetic material ofblack color need not be much contained in the toner particles, on theother hand, the two-component developer composed of a magnetic carrierand a non-magnetic toner can use a color toner having no color turbidityby the magnetic material so that a clean color image can be formed.

In the multiplex development, incidentally, it is sufficient to repeatseveral times the developments of the photosensitive member which hasalready been formed with the toner image. However, the multiplexdevelopment has problems that the toner image retained at a previousstep on the photosensitive member is disturbed upon development of asubsequent step, and that the toner having already been applied to thephotosensitive member is returned to a developing sleeve acting as adeveloper carrier until it steals into the developing means at asubsequent step, in which a developer in a color different from that ofthe developer of the previous step, thereby to cause color mixing. Inorder to obviate those problems, there is disclosed in Japanese PatentLaid-Open No. 144452/81, for example, means for superposing an a.c.component upon a developing bias while the photosensitive member beingout of contact with the developer layer on the developing sleeve actingas the developer carrier for developing an electrostatic latent image,except the developing means for first forming the toner image on thephotosensitive member. However, there arises a problem that the imagecan neither have a sufficient density nor be freed from the disturbanceor color mixing.

SUMMARY OF THE INVENTION

The present invention has been conceived so as to solve the aboveproblems which are concomitant with the image reproducing method of theprior art. A first object of the present invention is to a multipleximage reproducing method which is enabled to reduce the size of and thecost for a recording apparatus and to make easy and accurate thesynchronous control of image exposures by using a common apparatus forrepeatedly retaining electrostatic images. A second object of thepresent invention is to provide a multiplex image reproducing methodwhich can facilitate control of the adhesion of toners to electrostaticimages so that a sufficient developing density can be attained under anon-contact jumping developing condition to prevent any fog and themixture and application of the toners in different colors not only inthe case by the ordinary developing method for application the tonerscharged with an opposite polarity to the electrostatic images but alsoin the case by the forced developing method.

The present invention resides in a multiplex image reproducing method ofthe type, in which toner images are superposed on an image retainerhaving a photoconductive photosensitive surface layer by repeating theretentions and developments of electrostatic images on the imageretainer, characterized in that the retentions of said electrostaticimages are repeated by means of a common apparatus. The above-identifiedfirst object is achieved by the above-specified construction, and theabove-identified second object is achieved by using in the developingmeans the two-component developer which contains a mixture of a tonerand an insulating carrier.

Another object of the present invention is to provide a novel multipleximage reproducing method in which the retentions of electrostatic imagesare stabilized by using means for retaining the electrostatic images ona dielectric layer and in which a method of superposing developed imageson the dielectric layer is devised.

The above-specified object of the present invention can be achieved by amultiplex image reproducing method of the type, in which a plurality ofcolor toner images are superposed on an image retainer by repeating theretentions and developments of the electrostatic images on the imageretainer, characterized in that the retentions of said electrostaticimages are conducted on the dielectric surface layer.

Still another object of the present invention is to provide a multipleximage reproducing method of the type, in which a number of magneticrecording heads are juxtaposed to an image retainer formed of a magneticlayer and an insulating layer, if necessary, so that a number ofmagnetized regions may be formed on the magnetic layer of the imageretainer thereby to write an image by sending an image signal currentsynchronized with the movement of the image retainer to the respectiveones of said magnetic recording heads, while the image retainer beingmoved at a constant speed, whereby a toner image is obtained by applyinga magnetic toner to the written image to develop it, characterized inthat a plurality of toner images are formed in a superposed manner onthe image retainer by repeating the image writing and developingoperations. The developments are conducted under the condition in whichthe developer layers formed by the developing means are out of directcontact with the surface of the image retainer. Thus, there is no fearthat the toner images once formed are damaged upon the subsequentdevelopment so that the plural toner images can be superposed.

Moreover, the color images can be recorded by using the plural tonerimages in combination with toners of different colors such as yellow,cyan, magenta and black colors.

A further object of the present invention is to provide a multipleximage reproducing method which can not only obtain positive images andnegative images but also resort to the developing method, in which adeveloper charged with a polarity opposite to that of an electrostaticimage is applied to the electrostatic image by the coulomb force, sothat the application of the developer to the electrostatic image can bemade sufficient while reducing scattering of the developer.

The above-specified object of the present invention can be achieved by amultiplex image reproducing method of the type, in which toner imagesare superposed on an image retainer by repeating the retentions anddevelopments of electrostatic images upon the image retainer,characterized in that said electrostatic images are formed either byconducting image exposures after an image retainer formed with atransparent insulating surface layer on a photoconductive photosensitivelayer has been charged primarily and secondarily or by uniformlyexposing that image retainer after the image retainer has been subjectedto an image exposure simultaneously with the secondary charge.

More specifically, the multiplex image reproducing method of the presentinvention is different especially in the retentions of the electrostaticimages from those methods which are disclosed in Japanese PatentLaid-Open Nos. 144452/81, 116553/83 and 116554/83. In other words, themethod of the present invention is characterized in that the imageretainer is constructed to have the transparent insulating surface layerformed on the photoconductive photosensitive layer, and in that theelectrostatic images are formed either by primarily and secondarilycharging the image retainer and by subjecting the charged surface of thesame to the image exposure or by conducting the image exposuresimultaneously with the secondary charging treatment and by subsequentlyeffecting the uniform exposure. The above problems invited in themultiplex image reproducing method by the reversal developing method ofthe prior art can be eliminated by using the above-specified method ofretaining the electrostatic images.

A further object of the present invention is to provide a multipleximage reproducing method which can densely and finely reproduce a colorimage such as a landscape image, construct a reproducing apparatus in asmall size and at a low cost, and facilitate the synchronous control ofimage exposures.

The above-specified object of the present invention can be achieved by amultiplex image reproducing method of the type, in which a plurality ofcolor toner images are superposed on an image retainer by charging thesurface of the image retainer, by conducting the image exposures atleast repeatedly, and by conducting a development each time of the imageexposure by developing means, characterized: in that said imageexposures are conducted such that the spot position of a previous imageexposure and the spot position of a subsequent image exposure aresuperposed as the spot distribution exposure; and in that the imageexposures thus repeated are performed by means of a common apparatus.

A further object of the present invention is to provide a multipleximage reproducing method of reproducing images having a desirabledensity but neither disturbance nor color mixing by the use of adeveloper containing a plurality of components.

The above-specified object of the present invention can be achieved by amultiplex image reproducing method of the type, in which an image isretained on an image carrier by repeating a plurality of times both thestep of forming a latent image on said image carrier and the step ofdeveloping said latent image by the use of a developer having aplurality components, characterized in that the following relationshipsare satisfied:

    0.2≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦1.0,

wherein, at each developing step: the amplitude of the a.c. component ofa developing bias is designated by V_(AC) (V); the frequency of the sameby f (Hz); and the gap between said image carrier and a developercarrier for carrying said developer by d (mm).

More specifically, we, the Inventors, have researched the method ofretaining an image by conducting the development of the same while thea.c. component being superposed on the developing bias, and havediscovered that there is a region in which an image of high quality canbe obtained without incurring any disturbance and color mixing of theimage in accordance with the manner of selecting the developingconditions such as the a.c. bias or the frequency.

The present invention contemplates to provide a novel method which isbased upon the above-specified discovery.

Moreover, the above-specified object of the present invention can beachieved by the multiplex image reproducing method of the type, in whichan image is retained on an image carrier by repeating a plurality oftimes both the step of forming a latent image on said image carrier andthe step of developing said latent image, characterized in that thefollowing relationship is satisfied:

    0.2≦V.sub.AC /(d·f)≦1.6,

wherein, at each developing step: the amplitude of the a.c. component ofa developing bias is designated by V_(AC) (V); the frequency of the sameby f (Hz); and the gap between said image carrier and a developercarrier for carrying said developer by d (mm).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the construction of one embodiment ofthe reproducing apparatus for practising the method of the presentinvention:

FIG. 2 is a schematic view showing the construction of a laser beamscanner for image exposure;

FIG. 3 is a partially sectional view showing one example of developingmeans;

FIGS. 4 to 7 are flow charts for practising the methods of the presentinvention, respectively;

FIG. 8 is a partially sectional view showing the construction of animage retainer in another reproducing apparatus for practising themethod of the present invention;

FIG. 9 is a schematic view showing the construction of the reproducingapparatus;

FIG. 10 is a diagram schematically showing changes in the charged statesof one example of the process of electrostatic images;

FIG. 11 is a chart showing changes in the potential at the surfaceportion of the image retainer in a manner to correspond to FIG. 10;

FIGS. 12 to 17 are flow charts of practising the method of the presentinvention, respectively;

FIGS. 18 to 21 are flow charts showing another embodiment of the methodof the present invention to be practised by the reproducing apparatus ofFIG. 1, respectively;

FIGS. 22 to 25 are flow charts showing flow charts of an embodiment ofthe method of the present invention to be practised by the recordingapparatus of FIG. 9, respectively;

FIGS. 26 and 27 are schematic views showing an example of the recordingapparatus to be used for practising another embodiment of the method ofthe present invention, respectively; and

FIGS. 28 to 30 are flow charts for practising the method of the presentinvention, respectively.

In FIGS. 31 to 38 showing a further embodiment of the present invention:

FIG. 31 is a sectional view showing developing means and aphotosensitive drum;

FIGS. 32 and 33 are diagrams showing changes in the image density whenan a.c. current is changed;

FIG. 34 is a diagram showing the density characteristics when a fieldintensity and a frequency are changed;

FIGS. 35 and 37 are schematic views showing the essential portions ofthe multiplex image reproducing apparatus which are equipped with aplurality of developing means;

FIG. 36 is a chart showing changes in the surface potential of thephotosensitive drum which is used in the multiplex image reproducingapparatus of FIG. 35; and

FIG. 38 is a chart showing changes in the surface potential of thephotosensitive drum which is used in the multiplex image reproducingapparatus of FIG. 37.

FIGS. 39 and 40 are diagrams showing changes in an image density when ana.c. voltage applied to the developing means is changed in a furtherembodiment of the present invention;

FIG. 41 is a diagram showing the density characteristics when a fieldintensity and a frequency are changed;

FIGS. 42 and 43 are schematic views showing other examples of therecording means which are used for practising the method of the presentinvention, respectively; and

FIGS. 44 to 46 are flow charts for practising the method of the presentinvention, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail in the following inconnection with the embodiments thereof with reference to theaccompanying drawings.

FIG. 1 is a schematic view showing the construction of one example ofrecording apparatus for practising the method of the present invention;FIG. 2 is a schematic view showing a laser beam scanner for imageexposure; FIG. 3 is a partially sectional view showing one example ofdeveloping means; and FIGS. 4 to 7 are flow charts for practising themethod of the present invention, respectively.

In the recording apparatus of FIG. 1: reference numeral 1 is adrum-shaped image retainer which is formed with a photoconductivephotosensitive material such as Se and which is made rotatable in thedirection of arrow; numeral 2 is a charger for uniformly charging thesurface of the image retainer 1; numeral 3 is an exposing lamp foruniformly exposing to a weak optical ray the surface of the imageretainer which is used in the example of the flow chart of FIG. 7;numeral 4 is an image exposing ray of color images of different colors;numerals 5 to 8 are developing means using as their developers toners ofdifferent colors such as yellow, magenta, cyan or black; numerals 9 and10 are a pre-transfer charger and a pre-transfer exposing lamp which areprovided, if necessary, respectively, so that a color image retained onthe image retainer 1 with its plural color toner images being superposedmay be easily transferred to a recording member P; numeral 11 istransfer means; numeral 12 is fixing means for fixing the toner imagestransferred to the recording member P; numeral 13 is charge eliminatingmeans which is composed of a charge eliminator and/or a chargeeliminating corona discharger; and numeral 14 is cleaning means having acleaning blade or a fur brush which is adapted to come into contact withthe surface of the image retainer 1 after transfer of the color imagesfor eliminating the residual toners left on the surface and to leave thesurface of the image retainer 1 by the time the surface having beensubjected to a first development arrives.

Here, it is preferable to use as the charger 2 a corona discharger, asshown, which can apply such a stable charge as is hardly affected by aprevious charge, especially in case the surface of the image retainerhaving already been charged is to be additionally charged. In case thedrum-shaped image retainer 1 is used as in that reproducing apparatus,moreover, the image exposing ray 4 may be such an optical ray as hasbeen prepared by filtering a slit ray separately for colors, forexample, the optical ray of an ordinary monochromaticelectrophotographic reproducing machine. In order to reproduce a clearcolor image, however, an optical ray prepared by the laser beam scanner,as shown in FIG. 2, is preferable.

The laser beam scanner of FIG. 2 is formed into the image exposing ray 1for scanning the surface of the image retainer 4 at a constant speed byturning on or off the laser beam, which has emanated from a laser 21such as a He--Ne laser, by means of an acoustic-optical modulator 22 todeflect by means of a mirror scanner 23 composed of a rotating polygonalor optagonal mirror thereby to guide it through a focusing f-0 lens 24.Incidentally, reference numerals 25 and 26 indicate mirrors, and numeral27 indicates a lens for enlarging the diameter of a beam incident uponthe focusing f-e lens 24 so as to reduce the diameter of the beam on theimage retainer 1. If such a laser beam scanner as is shown in FIG. 2 isused for forming the image exposing ray 4, the electrostatic images canbe easily retained with a shift for different colors, as will bedescribed hereinafter, so that a clear color image can be reproduced.Despite of this fact, the image exposing ray 4 is not limited to theslit exposing ray or a dot exposing ray by the laser beam but may be onewhich is prepared by using an LED, a CRT, a liquid crystal or an opticalfiber transmitter, for example. In the reproducing apparatus in whichthe image retainer can take a planar state such as a belt shape,moreover, the image exposing ray may be a flash light.

As the developing means 5 to 8, on the other hand, there can bepreferably used those which have such a construction as is shown in FIG.3.

In FIG. 3: reference numeral 31 indicates a developing sleeve which ismade of a non-magnetic material such as aluminum or stainless steel;numeral 32 is a magnet which is equipped in the circumferentialdirection with a plurality of magnetic poles disposed inside of thedeveloping sleeve 31; numeral 33 is a layer thickness regulating bladefor regulating the thickness of a developer layer formed on thedeveloping sleeve 31; numeral 34 is a scraper blade for scraping thedeveloper layer after development from the surface of the developingsleeve 31; numeral 35 is an agitating rotor for agitating the developerin a developer reservoir 36; numeral 37 is a toner hopper; numeral 38 isa toner supply roller which is formed in its surface with a recess forreceiving the toner to supply the toner from the toner hopper 37 to thedeveloper reservoir 36; and numeral 39 is a power supply for applying abias voltage containing a vibratory voltage component, as the case maybe, to the developing sleeve 31 through a protecting resistor 40 togenerate an electric field for controlling the movements of the tonerbetween the developing sleeve 31 and the image retainer 1. FIG. 3 showsthat the developing sleeve 31 and the magnet 32 are rotatable in thedirections of arrows, respectively. It is, however, sufficient that thedeveloping sleeve 31 and the magnet 32 be fixed, or that the developingsleeve 31 and the magnet 32 be rotatable in a common direction. In casethe magnet 32 is fixed, it is customary to strengthen the magnetizationor to dispose two magnetic poles of identical or different polaritiesadjacent to each other so that the density of the magnetic flux of themagnetic pole facing the image retainer 1 may be stronger than that ofanother magnetic pole.

In these developing means, the magnetic poles of the magnet 32 areusually magnetized to a density of magnetic flux of 500 to 5,000 gaussesto attract the developer in the developer reservoir 36 to the surface ofthe developing sleeve 31 by that magnetic force so that the attracteddeveloper is formed, while having its thickness regulated by the layerthickness regulating blade 33, into a developer layer. This developerlayer is moved in the same direction as or in the opposite direction(although FIG. 3 shows the same direction) to the rotating direction ofthe image retainer 1, as indicated by the arrow, to develop theelectrostatic image of the image retainer 1 in the developing region, inwhich the surface of the developing sleeve 31 faces the surface of theimage retainer 1, whereas the residual is scraped away from the surfaceof the developing sleeve 31 by the scraper blade 34, until it isreturned to the developer reservoir 36. Moreover, the development, e.g.,at least the second or subsequent developments, which are repeated forsuperposing the color toner images, is preferred to be conducted underthe non-contact jumping developing condition so that the toner caught bythe image retainer 1 during the previous development may not be shiftedby the later development. FIG. 3 shows the state in which thedevelopment is executed under the non-contact jumping developingcondition.

Moreover, it is preferable to use in the developing means 5 to 8 theso-called "two-component developer" which is composed of a non-magnetictoner and a magnetic carrier and which is enabled to obtain a tonerimage of clear color without any necessity for containing a black orbrown magnetic material in the toner and to easily effect the control ofcharging the toner. Specifically, the magnetic carrier may preferably bean insulating carrier which has a resistivity of 10⁸ Ω cm or more or,preferably, 10¹³ or more and which is prepared either by dispersing andcontaining fine particles of a ferromagnetic or paramagnetic materialsuch as tri-ion tetroxide γ-ferric oxide, chromium dioxide, manganeseoxide, ferrite or manganese-copper alloy in a resin such as a styreneresin, a vinyl resin, an ethyl resin, a denaturated rosin resin, anacrylic resin, a polyamide resin, an epoxy resin or polyester resin, orby covering the surfaces of the particles of those magnetic materialswith the above-specified resins. If that resistivity is low, therearises such a problem, in case the bias voltage is applied to thedeveloping sleeve 31, that the charges are caused to migrate into thecarrier particles so that they become liable to be trapped by thesurface of the image retainer 1 and so that the bias voltage is notsufficiently applied. Especially, if the carriers are trapped by theimage retainer 1, the color image has its tone adversely affected.

Incidentally, the resistivity is a vlaue which is obtained by tappingthe particles in a container having an effective sectional area of 0.50cm², by subsequently loading the tapped particles with a load of 1Kg/cm², and by reading out a current value when a voltage for generatingan electric field of .[.1.000.]. .Iadd.1,000 .Iaddend.V/cm is appliedacross the load and the bottom electrode.

If the carriers have an average particle diameter less than 5 μm, on theother hand, the magnetization obtainable becomes too weak. If theaverage particle diameter of the carriers exceeds 50 μm, there arisetendencies that the image is not improved, and that a breakdown and adischarge become liable to occur so that a high voltage cannot beapplied. Therefore, the average particle diameter preferably has a valuemore than 5 μm and less than 50 μm, and a fluidizer such as hydrophobicsilica is suitable added as an additive, if necessary.

The toner may preferably be prepared by adding a variety of pigmentsand, if necessary, a charge controlling agent to a resin to have anaverage particle diameter of 1 to 20 μm and may preferably have anaverage charge of 3 to 300 μc/g or, especially, 10 to 100 μc/g. If thetoner has an average particle diameter smaller than 1 μm, it becomesreluctant to leave the carrier. If the average particle diameter exceeds20 μm, on the other hand, the image has its resolution degraded.

As the toner in the method of the present invention, there is used amagnetic or non-magnetic one which is used as an ordinary toner andwhich is prepared by dispersing a coloring agent if necessary and asuitable amount of magnetic material in a known resin. As the resin,there can be enumerated a synthetic resin such as: phenol, polystyrene,alkyd, polyacryl or polyethylene; polycarbonate, polyester, polyamide,polyether, polyolefin, polystyrene, a styrene-acrylate copolymer, astyrene-methacrylate copolymer, an unsaturated styrene-ethylenemonoolefin copolymer, styrene-vinylester copolymer, a styrene-vinylestercopolymer, a styrene-vinylether copolymer, a styrene-acrylonitrilecopolymer, a stryene-methacrylonitrile copolymer, a styrene-acrylamidecopolymer, a styrene-halogated vinylidene copolymer or polyvinylacetate; a binary, ternary or more copolymer of those; or a mixture ofthose copolymers.

As the coloring agent, in the other hand, there are enumerated a varietyof inorganic pigments, an organic pigment, a direct dye, an acid dye, abasic dye, a mordant, an acid mordant dye, a dispersed dye, anoil-soluble dye and so on. As a black pigment, more specifically, therecan be enumerated carbon black, acetylene black, lamp black, graphite,mineral black, anyline black, cyanine black and so on. As a yellowpigment, there can be enumerated chrome yellow, zinc yellow, bariumchromate, cadmium yellow, lead cyanamide, calcium plumbate, NaphtholYellow S, Hansa Yellow 10G, Hansa Yellow 5G, Hansa Yellow 3G, HansaYellow G, Hansa Yellow GR, Hansa Yellow A, Hansa Yellow RN, Hansa YellowR, Pigment Yellow L, Benzine Yellow, Benzine Yellow G, Benzine YellowGR, Permanent Yellow NCG, Vulcan Fast Yellow 5G, Vulcan Fast Yellow R,Tartrazine Yellow Lake, Quinoline Yellow Lake, Anthragen Yellow 6GL,Permanent Yellow FGL, Permanent Yellow H10G, Permanent Yellow HR,Anthrapyrimidine Yellow, and so on. As a red pigment, there can beenumerated a red iron oxide, red lead, silver vermilion, Cadmium Red,Permanent Red 4R, Para Red, polytungustophosphoriclacid, Fire Red,vermilion, Parachlor Orthonitroaniline Red, Lithol Fast Scarlet G.Brilliant Fast Scarlet, Brilliant Carmine BS, Permanent Red F2R,Permanent Red F4R, Permanent Red FRL, Permanent Red FRLL, Permanent RedF4RH, Fast Scarlet VD, Vulcan Fast Rubin B, eosine lake, Rhodamine Lake,Rhodamine Lake Y, Alyzarin lake, Thioindigo Red B, Thioindigo maroon,Permanent Red FGR, PV Carmine HR, and so on. As a blue pigment, therecan be enumerated ultramarine, prussian blue, cobalt blue, Alkali BlueLake, Peacock Blue Lake, Victoria Blue Lake, Metalless PhthalocyanineBlue, copper phthalocyanine, Fast Sky Blue, Indanthrene Blue RS,Indanthrene Blue BS, Indigo, and so on. As a yellow dye, there can beenumerated C.I. (i.e., Color Index) Direct Yellow 98, C.I. Direct Yellow89 and C.I. Direct Yellow 88 (all of which are of direct type), C.I.Acid Yellow 1, C.I. Acid Yellow 3 and C.I. Acid Yellow 7 (all of whichare of acid type), C.I. Basic Yellow 1, C.I. Basic Yellow 2 and C.I.Basic Yellow 11 (all of which are of basic type), C.I. Modern Yellow 26(which is of mordant or acid mordant type), C.I. Disperse Yellow 1, C.I.Disperse Yellow 3 and C.I. Disperse Yellow 4 (all of which are ofdisperse type), C.I. Solvent Yellow 2, C.I. Solvent Yellow 6 and C.I.Solvent Yellow 14 (all of which are of oil soluble type), and so on. Asa red dye, there can be enumerated C.I. Direct Red 1, C.I. Direct Red 2and C.I. Direct Red 4 (all of which are of direct type), C.I. Acid Red8, C.I. Acid Red 13 and C.I. Acid Red 14 (all of which are of acidtype), C.I. Basic Red 2, C.I. Basic Red 14 and C.I. Basic Red 27 (all ofwhich are of basic type), C.I. Modern Red 21 (which is of mordant oraxid mordant type), C.I. Disperse Red 1, C.I. Disperse Red 4 and C.I.Disperse Red 5 (all of which are of disperse type), C.I. Solvent Red 1,C.I. Solvent Red 3 and C.I. Solvent Red 8 (all of which are ofoil-soluble type), and so on. As a blue dye, there can be enumeratedC.I. Direct Blue 1, C.I. Direct Blue 6 and C.I. Direct blue 22 (all ofwhich are of direct type), C.I. Acid Blue 1, C.I. Acid Blue 7 and C.I.Acid Blue 22 (all of which are acid type), C.I. Basic Blue 7, C.I. BasicBlue 9 and C.I. Basic Blue 19 (all of which are of basic type), C.I.Modern Blue 48 (which is of mordant or acid mordant type), C.I. DisperseBlue 1, C.I. Disperse Blue 3 and C.I. Disperse Blue 5 (all of which areof disperse type), C.I. Solvent Blue 2, C.I. Solvent Blue 11 and C.I.Solvent Blue 12 (all of which are oil-soluble type), and so on. However,the coloring agent should not be limited to those thus far enumerated.Moreover, the colors of the toners should not be limited to theabove-specified four but can be freely selected in accordance with theobject of use.

As the magnetizing material for magnetizing the toner, there can be useda material which is similar to that used in the carrier. The amount ofaddition of the magnetic material is preferred to be smaller than 60 wt.% of the toner and especially preferred to be up to 30 wt. % so that theclearness of the color of the toner may not be deteriorated.

In order to improve the clearness of the color of the toner, moreover,it is possible to use a coloring magnetic material or a transparentmagnetic material using a rare earth element. As a suitable one for thecoloring magnetic material, there can be enumerated: for a red color, aniron oxide (e.g., a red oxide), a material prepared by covering thesurface of Ni with a copper oxide, or a material prepared by causing Nito absorb Cadmium Red; for a blue color, cobalt or its compound; and fora yellow color, aniron oxide or a material prepared by causing Ni toabsorb Cadmium Yellow.

Moreover, it is quite natural that there can be applied to theabove-specified toner a variety of known additives, which are usuallyused in the toner, such as a charge control agent.

On the other hand, the toner to be used in the present invention ispreferred to have an average particle diameter of 1 to 20 μm and anaverage charge of 1 to 300 μc/g or, especially preferably, 3 to 30 μc/g.If the toner has an average particle diameter smaller than 1 μm, itbecomes reluctant to leave the carrier. If the average particle diameterexceeds 20 μm, on the other hand, the resolution of the image isdegraded.

If the developer composed of a mixture of the insulating carrier and thetoner thus for described, it is possible to easily set the bias voltage,which is to be applied to the developing sleeve 31 of FIG. 3, withoutany fear of leakage such that the toner is sufficiently applied to theelectrostatic image but without any fog. Incidentally, in order to makemore effective the control of development and movement of the toner bythe application of such bias voltage, the magnetic material to be usedin the magnetic carrier may be contained in the toner within such arange as will not deteriorate the color clearness.

The descriptions made hereinbefore are directed the constructions of tothe developing means and the developer which are preferably used in themethod of the present invention. However, the present invention is notlimited to them but may use such developing means and developer as aredisclosed in Japanese Patent Laid-Open Nos. 30537/75, 18656 to 18659,80,144452/81, 116553/83, and 116554/83. More preferably, there may beresorted to such a non-contact jumping developing condition by atwo-component developer as is disclosed in Japanese Patent Laid-OpenNos. 57446/83, 96900 to 96903/83 and 97973/83.

By the use of the recording apparatus thus far described, the method ofthe present invention, as shown in FIGS. 4 to 7, can be put to practice.Incidentally, all of FIGS. 4 to 7 show the step at which the.[.development of FIG. 2.]. .Iadd.latent image formation with theapparatus .Iaddend.has been conducted.

FIG. 4 shows an embodiment of the present invention, in which an exposedportion PH provides a background whereas an unexposed portion DA isformed with an electrostatic image by an electrostatic image retainingmethod and in which the development is conducted by applying to theelectrostatic image a toner charged with an opposite polarity. This isthe embodiment having one color image reproducing cycle comprising,according to the reproducing apparatus of FIG. 1, the steps of:uniformly conducting a first charge of the surface of the image retainer1 in its initial state, in which the image retainer 1 is cleared of acharge by the charge eliminating means 13 and cleaned by the cleaningmeans 14 to a potential 0, at its one rotation by means of the charger2; executing a first image exposure of the charged surface by means ofthe image exposing ray 4 of each of different colors so that thepotential other than the electrostatic image portion may besubstantially 0; conducting a first development of the electrostaticimage, the resultant potential of which is substantially equal to thatof the first charge, by means of such one of the developing means 5 to 8as uses a developer having a color toner corresponding to the imageexposing ray 4; uniformly eliminating the charge to return again thesurface potential of the image retainer 1 to zero by means of the chargeeliminating means 13 (or only its charge eliminating lamp) because thepotential of the electrostatic image, which is dropped as a result thatit traps the toner T charged with the opposite polarity, is still higherthan that of the background; uniformly conducting a second charge againat a second rotation by means of the charger 2; conducting such a secondimage exposure of the secondly charged surface by means of the imageexposing ray 4 having a color different from the previous one as willdrop the potential at other than the electrostatic image portionsubstantially to zero; conducting a second development of theelectrostatic image obtained with a toner T' by means of anotherdeveloping means using a developer containing a color tonercorresponding thereto; subsequently repeating third and fourthretentions and developments of the electrostatic image in a similarmanner; operating the pre-transfer charger 9 and the pre-transferexposing lamp 10 until the color image, which has been subjected to thefourth development so that the color toner images are superposed, movestherethrough after that color image has been retained; next transferringthe color image to the member P, which is being fed in synchronism withthe rotations of the image retainer 1, by means of the transfer means11; fixing the color image transferred to the recording member P bymeans of the fixing means 12; eliminating the charge from the surface ofthe image retainer 1 having the transferred color image by means ofcharge eliminating means 13; and cleaning the charge-removed surface ofthe image retainer 1 by means of the cleaning means 14 until the initialstate is restored to complete the one color image reproducing cycle.More specifically, the charging operation for each retention of theelectrostatic image is performed by the charger 2, and the imageexposure is performed by means of the common slit exposing means, whichis equipped with filter switching means, or the common exposing meanswhich is constructed of the laser beam scanner of FIG. 2, for example.This makes it unnecessary to use another image exposing means forretaining the electrostatic image at each time so that the reproducingapparatus can be constructed in a small size and at a low cost and sothat the synchronous control of the retention of the electrostatic imageat each time can be conducted with ease. Incidentally, the chargeelimination by the charge eliminating means 13, which is interposedbetween a previous development and a subsequent charging operation, canbe omitted.

Since the development is effected by the method of developing theelectrostatic image with the toner charged with the opposite polarity,the embodiment of FIG. 4 being described can easily enhance thedeveloped densities of the respective colors so that a clear color imagecan be easily reproduced. Incidentally, in order to avoid the colormixing, the d.c. biases at the developments may be set a consecutivelyhigher levels at the later steps. In a matter to correspond to thissetting, moreover, the charged potentials may be set at consecutivelyhigher levels.

FIGS. 5 to 7 show other embodiments of the present invention, in whichelectrostatic images are retained by an electrostatic retaining methodof forming the image exposed portion PH into an electrostatic image at alower potential than the background portion and in which thedevelopments are conducted by applying to the electrostatic images thetoners for charging the same with the same polarity as that of thebackground potential.

The embodiment of FIG. 5 by the reproducing apparatus of FIG. 1 has onecolor image reproducing cycle comprising the steps of: uniformlycharging the surface of the image 1 in the same initial state as that ofFIG. 4 at its one rotation by means of the charger 2; projecting theimage exposing rays 4 of different colors by the laser beam scanner ofFIG. 2 onto the charged surface to conduct a first image exposurethereby to drop the potential of the electrostatic image portionsubstantially to zero; conducting a first development of the obtainedelectrostatic image by such any one of the developing means 5 to 8 asuses the developer (of which the toner charges the image retainer 1 withthe same polarity, as is different from the embodiment of FIG. 4) havingthe color toner corresponding to the image exposing ray 4; conducting asecond image exposure projecting the image exposing ray 4 of a differentcolor onto a position, which is shifted from the projected position ofthe previous developing ray 4, at a second rotation by means of the samelaser beam scanner without any use of the charger 2; developing theelectrostatic image thus obtained to have a substantially zero potentialwith a developer having a toner of corresponding color; and subsequentlyrepeating the third and fourth retentions and developments of theelectrostatic image in like manners thereby to subsequently complete theone color image reproducing cycle similarly to FIG. 4. In thisembodiment, incidentally, even if the electrostatic image having thesubstantially zero potential is developed so that it traps the toner Tfor effecting the charge with the same polarity as that of the charge ofthe image retainer 1, the potential is not substantially equal to thatof the background portion, as shown. As a result, upon the developmentfor applying the toner T' of different color to the electrostatic latentimage retained later, the toner T' is frequently applied in a superposedmanner to the electrostatic image portion having previously trapped thetoner T, although the portion is neither exposed nor written in yet.Since the laser beam scanner is used for preparing the image exposingray 4, however, the projection position of the image exposing ray 4 ateach time is so remarkably simple that the charging operation can befinished at one time. Moreover, it can be prevented by setting the d.c.biases of the respective developments at consecutively lower levels thatthe electrostatic images of different colors are liable to besuperposed, whereby an excellently clear color image can be obtained.

The embodiment of FIG. 6 is one which is improved in such defects of theembodiment of FIG. 5 that the electrostatic image cannot be positivelyretained in the superposed manner at the position where theelectrostatic image has been previously formed, and that there is afear, to the contrary, that a toner of different color may be applied,even if a little, by the subsequent development to the electrostaticimage portion which has been previously developed. More specifically,the embodiment of FIG. 6 is identical, at the steps from the initialtime to the first development, to the steps up to the first developmentof FIG. 5 but is different from the same in the steps of: subsequentlyconducting or not the charge elimination by means of the chargeeliminating means 13 (or only the charge eliminating lamp); uniformlyconducting the second charge again at the second rotation by means ofthe charger 2; conducting the second image exposure and the seconddevelopment of the charged surface; and subsequently repeating the thirdand fourth retentions and developments of the electrostatic image in asimilar manner. Thus, in the embodiment of FIG. 6, in which the surfaceof the image retainer 1 is uniformly charged again after the previousdevelopment and is then subjected to the subsequent electrostatic imageretentions and developments, there can be attained the effects that theelectrostatic image can be retained in the superposed manner on theposition where the electrostatic image has been previously retained,like the embodiment of FIG. 4, and that, in case the position of thesubsequent electrostatic image is shifted from that of the previous one,the subsequent toner of different color is hardly applied to the imageposition to which the previous toner is applied.

The embodiment of FIG. 7 is one which is especially devised to prevent asubsequent toner of different color from being applied to an imageposition to which a previous toner has been applied. This example isidentical in the steps before a first development to the embodiments ofFIGS. 6 and 7 but is different from the same in the steps of: eitherpreviously uniformly exposing, after the first development, the surfaceof the image retainer 1 by the user of the pretransfer exposing lamp 10or the charge-eliminating lamp of the charge-eliminating means 13, andsubsequently conducting a second charge by means of the charger 2, orpreviously uniformly conducting a second charge by means of the charger2, and subsequently uniformly conducting a weak exposure by means of theexposing lamp 3; then conducting a second image exposure and a seconddevelopment; and repeating third and fourth retentions and developmentsof electrostatic images in a similar manner. Here, if the uniformexposure is previously conducted after the development, the portionwhich has been developed to trap the toner has its charge uneliminatedso that it is held at a high potential, whereas the remaining portion isdropped substantially to a zero potential, whereupon the potential atthe portion having the toner can be raised by conducting the secondcharge to a level slightly higher than that at the other portion, wherethe electrostatic image is to be retained, thereby to charge the surfaceof the image retainer 1. On the other hand, even if the second charge ispreviously conducted after the development to uniformly charge thesurface of the image retainer 1 whereupon the weak exposure is uniformlyconducted, the charged state of the surface of the image retainer 1becomes similar to that obtainable in case the uniform exposure has beenpreviously conducted. As a result, when the subsequent electrostaticimage retained with a shift of position is to be developed, the previousportion having the toner is at a higher potential so that it can beeffectively prevented from trapping the toner of different color.

In any of the foregoing embodiments, it is preferred that the developingmeans 5 to 8 use the developer which is composed of a mixture of thetoner and the insulating carrier, and that the development is conductedunder the non-contact jumping developing condition. As a result, as hasalso been described, it becomes possible to prevent the mixing of thetoner of different color and to easily apply the bias voltage suitablefor the toner control, whereby a color image having a high developeddensity and an excellent clearness can be reproduced even in the case ofthe electrostatic image retaining method and the developing method, asin the embodiments of FIGS. 5 to 7, in which the image exposing meanssuch as the laser beam scanner can be advantageously used.

Next, the embodiments of FIGS. 4 to 7 will be described morespecifically in the following in connection with Examples 1 to 4,respectively.

EXAMPLE 1 (I.E., EMBODIMENT OF FIG. 4)

The reproducing apparatus, as shown in FIG. 1, was used. However, theexposing lamp 3 was not used, but the image retainer 1 had a surfacelayer of a photosensitive material such as CdS and a circumferentialspeed of 180 mm/sec. The surface of the above-specified image retainer 1was charged with a voltage of -500 V by means of the charger 2 using thecorona discharger, and its charged surface was subjected to the slitexposure through a blue filter. As a result, there was retained in theimage retainer 1 the electrostatic image in which the exposed portion PHhad the background potential of -50 V whereas the unexposed portion DAhad the potential of -500 V. The electrostatic image thus retained wasfirstly developed by the developing means 5, as shown in FIG. 3.

The developing means 5 used the developer, which was composed of: acarrier prepared by dispersing and containing 50 wt. % of magnetite in aresin to have an average particle diameter of 20 μm, a magnetization of30 emu/g and a resistivity of 10¹⁴ Ω cm or higher; and a non-magnetictoner prepared by adding 10 wt. parts of a benzine derivative as theyellow pigment and another charge controlling agent to the styreneacrylresin to have an average particle diameter of 10 μm, under a conditionthat the ratio of the toner to the carrier was 25 wt. %. Moreover, thenon-contact jumping developing conditions was resorted to, under whichthe developing sleeve 31 had an external diameter of 30 mm and a numberof revolutions of 100 r.p.m., under which the magnet 32 has its N and Smagnetic poles of a magnetic flux density of 1,000 gausses and a numberof revolutions of 1,000 r.p.m., under the layer of the developer in thedeveloped region had a thickness of 0.7 mm, under which the gap betweenthe developing sleeve 31 and the image retainer 1 was 0.8 mm, and underwhich a superposed voltage containing a d.c. voltage of -100 V and ana.c. voltage of 3 kHz and .[.1.000.]. .Iadd.1,000 .Iaddend.V was appliedto the developing sleeve 31.

While the developing image was being developed by the developing means5, the remaining developing means 6 to 8, as shown in FIG. 3, were heldin their undeveloping state. This was achieved by disconnecting thedeveloping sleeve 31 from the power supply 39 into its floating state,by grounding the same to the earth, or by positively applying the d.c.bias voltage, which had the same polarity as the charge of the imageretainer 1 but the opposite polarity to the charge of the toner, to thedeveloping sleeve 31. Of these, it is preferred to apply the d.c. biasvoltage. Since the developing means 6 to 8 were made to conduct theirdevelopments under the non-contact jumping developing condition like thedeveloping means 5, it was not necessary to especially eliminate thelayer of the developer from the developing sleeve 31. Of thosedeveloping means 6 to 8: the developing means 6 used a developer whichwas prepared by replacing the toner of the developer of the developingmeans 5 by a toner containing polytungstate as the Magenta pigment inplace of the yellow pigment; the developer 7 used a developer which wasprepared by replacing the same toner by a toner containing copperphthalocyanine as the cyan pigment; and the developing means 8 used adeveloper which was prepared by replacing the same toner by a tonercontaining carbon black as the black pigment. It is quite natural that atoner containing other pigment and dye could be used as the color toner,and that the order of the colors to be developed and the order of thedeveloping means could be suitably selected.

The surface of the image retainer 1, which had been subjected to thefirst development, was recharged to -600 V by operating the chargeeliminating means 13 and the charger 2 (although the former may be leftinoperative). The charged surface was subjected to the second exposureby the slit exposure through a green filter, and the developing sleeve31 was then subjected to the second development with the Magenta tonerby the developing means under the non-contact jumping developingcondition for applying the superposed voltage of a d.c. voltage of -200V and an a.c. voltage of 3 kHz and 1,000 V. Likewise, the followingsteps were repeated: the charge, the slit exposure through a red filter,and the third development of the cyan toner by the developing means 7;and the charge, the slit exposure without any filtration, and the fourthdevelopment of a black toner by the developing means 8. In the secondand later developments, incidentally, the amplitudes and frequencies ofthe d.c. bias component and a.c. component of the voltage to be appliedto the developing sleeve 31, the selecting time of a time selectingconversion, and so on were suitably changed in accordance with thechanges in the surface potential, the developing characteristics and thecolor reproductivity of the image retainer 1. Especially, it waseffective for preventing the color mixing of the toner that the chargepotentials were made consecutively higher whereas the d.c. biases weremade consecutively higher.

The color images of four colors were made liable, when they wereretained on the image retainer 1 as a result of the fourth development,to be transferred by the pre-transfer charger 9 and the pre-transferexposing lamp 10, and were than transferred to the recording member P bythe transfer means 11 until they were fixed by the fixing means 12. Theimage retainer 1 having the color images transferred thereto had itscharge eliminated by the charge eliminator 13 and its residual tonerscraped off from its surface by the contacts of the cleaning blade orfur brush of the cleaning means 14. At the instant when the surfacehaving the color images retained therein passed through the cleaningmeans 13, the one color image reproducing cycle was completely ended.

The color image thus reproduced was so clear that the respective colorsexhibited sufficient densities, but a slight color mixing of the tonerswas found at the portion in which the respective color toners aredensely trapped.

EXAMPLE 2 (I.E., EXAMPLE OF FIG. 5)

The reproducing apparatus, as shown in FIG. 1, was used. However, theexposing lamp 3 was not used, but the image retainer 1 had a surfacelayer of a photosensitive materail such as CdS and a circumferentialspeed of 180 mm/sec. The surface of the above-specified image retainer 1was charged with a voltage of +500 V by means of the charger 2 using thecorona discharger, and its charged surface was subjected to the firstimage exposure in a density of 16 dots/mm by the laser beam scanner ofFIG. 2 using a He--Ne laser. As a result, there was retained in theimage retainer 1 the electrostatic image in which the exposed portion PHhad a potential of +50 V contrary to the background potential of +500 V.The electrostatic image thus retained was firstly developed by thedeveloping means 5, as shown in FIG. 3.

Incidentally, the developing conditions by the developing means 5 weremade identical to those of Example 1 except that the carrier of thedeveloper had an average particle dize of 30 μm, that the ratio of thetoner to the carrier was 20 wt. %, and that the superposed voltage of ad.c. voltage of +400 V and an a.c. voltage of 1.5 kHz and 1,000 V wasapplied to the developing sleeve 31. Moreover, the conditions of theremaining developing means 6 to 8 were identical to those of the Example1 except the bias voltages. In this case, however, the bias voltages forholding the developing means taking no part in the development in thestate other than the development had a polarity opposite to both thoseof the charge of the toner and the charge of the image retainer 1.

The surface of the image retainer 1 having been subjected to the firstdevelopment was subjected again to the second image exposure without anychange in the density but with a shift of the dot position by means ofthe same laser beam scanner without operating one of the pre-transfercharger 9, the pre-transfer exposing lamp 10, the charge eliminatingmeans 13, the cleaning means 14 and the charger 2, and was thensubjected to the second development with the Magenta toner by thedeveloping means 6. Likewise, the third development with the cyanictoner by the developer means 7 and the fourth development with the blacktoner by the developing means 8 were repeated. In the second and laterdevelopments, incidentally, the amplitudes and frequencies of the d.c.bias component and a.c. component of the voltage to be applied to thedeveloping sleeve 31, the selecting time of a time selecting conversion,and so on were suitably changed in accordance with the changes in thesurface potential, the developing characteristics and the colorreproductivity of the image retainer 1. Especially, in this case it waseffective for preventing the color mixing of the toner that the d.c.biases were made consecutively lower at the respective steps.

After the fourth development had been conducted to retain the colorimage of four colors on the image retainer 1, it was transferred andfixed to the recording member P like the Example 1, and the imageretainer 1 had its charge eliminated and was cleaned, thus ending theone color image reproducing cycle.

The color image thus reproduced was clear like that of the Embodiment 1.

EXAMPLE 3 (I.E., EMBODIMENT OF FIG. 6)

By using the same apparatus as that of the Example 2, the color imagereproduction was performed under the same conditions as those of theExample 2 except that the voltage to be applied to the developing sleeve31 of the developing means was a superposed voltage of a d.c. voltage of+400 V and an a.c. voltage of 500 Hz and 250 V, that the chargeelimination was conducted by means of the pre-transfer exposing lamp 10before each image exposure on and after the second image exposing step,and that the surface potential of the image retainer 1 was thenrecharged with +500 V by the charger 2.

The color image reproduced was more clear than that obtained by theExample 2, because the color mixture of the toners was reduced at theportion where the respective color toners were densely trapped.

According to this Example, incidentally, the previous image exposedposition and the subsequent image exposed position can be superposed, ashas been described hereinbefore, and, in this case, the order of thecolors to be developed imparts considerable influences upon theclearness of the color image. It is, therefore, necessary to determineespecially carefully the order of the colors to be developed.

EXAMPLE 4 (I.E., EMBODIMENT OF FIG. 7)

By using the same apparatus as that of the Example 2 except theprovision of the exposing lamp 3, the color image reproduction wasconducted under the same conditions as those of the Example 2 exceptthat the voltage to be applied to the developing sleeve 31 of thedeveloping means was a superposed one of a d.c. voltage of +450 V and ana.c. voltage of 2 kHz and 500 V, and that both the charge by the charger2 for raising the surface potential of the image retainer 1 to +600 Vand the uniform, weak exposure by the exposing lamp 3 for dropping thesurface potential to +500 V were conducted before each image exposure onand after the second image exposure.

The color image thus reproduced has no color mixing of the respectivecolor toners even at a portion, where the toners were densely trapped,so that it was remarkably clear.

Even in this Example, like the Example 3, the previous image exposedposition and the subsequent image exposed position can be superposed.

According to the present invention, the common apparatus can be used forthe repeated electrostatic image retentions to provide excellent effectsthat the reproducing apparatus can be constructed in a small size and ata low cost, and that the synchronous control of each image exposure canbe conducted easily and accurately. Moreover, each development can beconducted either by the developing method, in which there is applied toan electrostatic image relatively easy to have its developed densitycontrolled to a toner for charging it with the opposite polarity, or bythe developing method in which the laser beam scanner can be used as theimage exposing means and in which there is applied to the electrostaticimage a toner for charging it with the common polarity. According toeither of the developing methods, still moreover, there can be attainedan excellent effect that the development can be conducted under thenon-contact jumping developing condition to reproduce the color imagehaving a sufficient developed density and an excellent clearness.

As has been described hereinbefore, incidentally, the present inventionshould naturally be limited neither to the method, in which the imageretained uses the drum-shaped recording apparatus, but also to themethod in which the color image is transferred to the recording member.In other words, the present invention can be applied to the method, inwhich the image retainer is mounted on a base such as that forelectrofax paper and in which the color image retained thereon is nottransferred but fixed. In this application, there can be dispensed withthe pre-transfer charger, the pre-transfer exposing lamp, the transfermeans and the cleaning means. It is true, but the pre-transfer charger,the pre-transfer exposing lamp or the charge eliminating means can beomitted in the transfer case, too, and the transfer may be a directpressure one or one using an intermediate transfer member whereas thefixing operation should not be limited to one using a heat roller.

In other embodiments of the present invention, the image retainer 1 isconstructed, as shown in FIG. 8, of: a conductive base 1a made ofaluminum or nickel; a photoconductive photosensitive layer 1b made ofSe, CdS, Si or the like and formed on the conductive base 1a; and atransparent insulating surface layer 1c made of a transparent resin andformed on the photoconductive photosensitive layer 1b, and saidconductive substrate 1a being grounded to the earth. Indicated atreference numeral 2 in FIG. 9 is a primary charger which is composed, incombination, of a lamp 2a for irradiating the surface of the imageretainer 1 and a corona discharger 2b. Numeral 3' indicates a secondarycharger which is composed of a corona discharger. Here, the primarycharger 2 need not necessarily be equipped with the lamp 2a in case thephotoconductive photosensitive layer 1b of the image retainer 1 has suchsemiconductor characteristics as exhibit a rectifying action thatcharges can be implanted from the base 1a. Indicated at numeral 15 is acorona discharger for charging toners prior to transfers.

In the reproducing apparatus thus constructed, when the primary charger2 conducts the corona discharge by its corona discharger 2b whileirradiating the surface of the image retainer 1 by its lamp 2a (whichmay not be required as the case may be, as has been describedhereinbefore), the image retainer 1 is charged, as shown in FIG. 10A, inits photoconductive photosensitive layer 1b and on the surface of itstransparent insulating surface layer 1d. When the secondary charger 3'conducts the corona discharge of the charged surface of the imageretainer 1, the charges on the surface of the transparent insulatingsurface layer 1c are reduced because the photoconductive photosensitivelayer 1b has an insulating property in this case, so that the chargedstate of the image retainer 1 charges as shown in FIG. 10B. When theimage exposing ray 4 is incident upon that surface of the image retainer1 thus secondarily charged, the surface charges of the photoconductivephotosensitive layer 1b at the exposed portion PH are reduced whilebeing left as they are at the unexposed portion DA so that the chargesof the image retainer 1 change, as shown in FIG. 10C. The changes of thesurface potential of the image retainer 1 in this meanwhile are shown inFIG. 11, in which the potentials in states A, B and C correspond to thechanged states of FIGS. 10A, B and C, respectively. More specifically,the potential of the exposed portion PH exposed to the image exposingray 4 takes such surface one as is indicated at C(PH) in FIG. 11,whereas the potential of the unexposed portion having received none ofthe image exposing ray 4 is at C (DA) which is substantially the same asthe surface potential indicated at B in FIG. 11. This results in that anelectrostatic image having a surface potential at C(PH) with respect tothat background is retained by the image exposing ray 4. Theelectrostatic image thus retained can be developed like the ordinaryelectrophotographic reproducing machine with such a developer as willcharge the exposed portion PH with a polarity opposite to that of thelatent image, and this development is conducted by any one of such acolor of the developing means 5 to 8 as corresponds to the imageexposing ray 4 having formed the electrostatic image. When thedevelopment is conducted to apply the toner, the potential of theelectrostatic image is dropped in accordance with the trapped amount ofthe toner having been charged in the opposite polarity. However, thecharge of the unexposed portion DA of the image retainer 1, which hasreceived none of the image exposing ray 4, remains in such a state DA ofFIG. 10C as is identical to that of FIG. 10B, and the surface potentialof the same portion is at the same potential C(DA) as that of B of FIG.11 even with a dark attenuation. As a result, if the image exposing ray4 of another color is incident with a shift from the position of theprevious image exposing ray 4 at the second rotation of the imageretainer 1, an electrostatic image can be retained like the previousexposure with neither of the primary and secondary charging operations.Thus, the second and later retentions of the electrostatic images bymaking use of the first primary and secondary charging operations areeasily effected in case the previous development or developments areconducted by the developing method, in which a developer for charging inan opposite polarity is applied, and under the noncontact jumpingcondition. This is partly because the application of the toners to theelectrostatic images can be more easily conducted than the developingmethod, in which the developing agent for charging in the same polarityto make it unnecessary to apply such a high voltage to the developingsleeve 16 as to apply the toners to the electrostatic images so that thecharged state of the image retainer 1 is held stable, and partly becausethe developer layer of the developing means is kept away from thesurface of the image retainer 1 by resorting to the non-contact jumpingdeveloping condition so that the charged state of the image retainer 1is held stable. Incidentally, the shift of the position of thesubsequent image exposing ray 4 with respect to the previous imageforming ray 4 so as to retain the electrostatic images of differentcolors can be easily conducted by using the laser beam scanner of FIG. 3for producing the image exposing ray 4.

The present invention should not be limited to the embodiment in whichthe second and later retentions of the electrostatic images areconducted by making use of the first primary and secondary chargingoperations. Generally speaking, however, the primary and secondarycharging operations may be conducted each time of the second and laterretentions of the electrostatic images, or therefore the previouscharges may be eliminated by the charge eliminating means 13 or only thesecondary charging operation may be so executed as to compensate thedark attenuation. Especially either in case the primary and secondarycharging operations are executed again without eliminating the previouscharges or in case only the secondary charging operation is executed,the scorotron corona discharger may preferably for allowing stablecharging operations even in the presence of the previous charges maypreferably be used as the corona dischargers of the primary charger 2and the secondary charger 3'. It is especially desired when a gradedreproductivity is stressed or in case the image exposing ray 4 is ofslit or flash exposure type that the primary and secondary chargingoperations are conducted again each retention of the electrostaticimages. Moreover, it is possible to adopt the NP or KIP method by whichthe electrostatic images are retained by conducting the image exposuresimultaneously with secondary charging operation after the primarycharging operation and by conducting the exposure of the whole surface.In the several methods thus far described, the electrostatic imagepotential can be so controlled in dependence upon the relative strengthsof the primary and secondary charges that the exposed portion and theunexposed portion may be in an identical or opposite polarity.Considering the feasibility of the development, however, it ispreferable that the exposed and unexposed portions take oppositepolarities.

The electrostatic image at the second rotation thus retained isdeveloped by such one of a color different from the previous one of thedeveloping means 5 to 8 as corresponds the image exposing ray 4 havingretained that electrostatic image. Likewise, at the third and fourthrotations of the image retainer 1, too, both the retentions of theelectrostatic images and the developments by the different developingmeans corresponding to those electrostatic images are conducted to forma color image which has toner images of different colors superposed onthe image retainer 1. Moreover, the surface of the image retainer 1having been subjected to the last development has, if necessary, chargesapplied to its toner image by means of the corona discharger 15 and isthen irradiated by the pre-transfer lamp 10 so that the color image canbe easily transferred to the recording member P by the transfer means11. The color image thus transferred to the recording member P is fixedto the recording member P by the fixing means 12. The surface of theimage retainer 1 having the color image transferred thereto has itscharges eliminated by the charge eliminating means 13 so that theresidual toners after the transfer are eliminated as a result that thecleaning blade of the cleaning means 14 having been kept away until thetime comes into abutment against the surface of the image retainer 1. Atthe time the surface portion of the image retainer 1 having retained thecolor image moves over the cleaning means 14, the cleaning blade leavesthe surface of the image retainer 1, thus completing the one color imagereproducing cycle.

FIG. 12 is a flow chart showing the changes of the surface potential ofthe image retainer of the embodiment of the present invention, in whichan electrostatic image is retained each time with an image exposing rayby making use of the first primary and secondary charging operations.FIG. 13 is a flow chart showing the changes of the surface potential ofthe image retainer of the embodiment of the present invention, in whichonly the secondary charging operations is conducted prior to the secondand later electrostatic image retentions. FIG. 14 is a flow chartshowing the changes of the surface potential of the image retainer ofthe embodiment of the present invention, in which the primary andsecondary charging operations are conducted after charges are eliminatedfrom the second and later retained electrostatic images like the firstone. All of these Figures show, like FIGS. 10 and 11, the embodiments ofthe present inventions for retaining the electrostatic images by thenegative exposure, where the image exposed portion retains theelectrostatic images, to which the toners are applied. On the contrary,FIG. 15 is a flow chart showing the changes of the surface potential ofthe image retainer of the embodiment of the present invention forretaining the electrostatic images by the positive exposure, where theexposed portion is formed into the background portion whereas theunexposed portion is formed into the electrostatic image, to which thetoners are applied, by changing the conditions for the primary andsecondary charging operations in the embodiment of FIG. 14. On the otherhand, FIGS. 16 and 17 are flow charts showing the changes of the surfacepotentials of the image retainers of the embodiments of the presentinvention using the electrostatic image retaining process called the "NPor KIP process". FIG. 16 corresponds to the case of the positiveexposure like FIG. 15, whereas FIG. 17 corresponds to the case of thenegative exposure like FIGS. 12 to 14. The embodiments of FIGS. 16 and17 will be briefly described in the following. FIG. 16 shows theembodiment comprising the steps of: imparting charges to thephotoconductive photosensitive layer 1b by the primary exposingoperation to charge the image retainer 1 positive; subsequentlyconducting an image exposure in a secondary charging operation to forman ion image on the transparent insulating surface layer 1c thereby tocharge the image retainer 1 negative; subjecting the whole surface to anexposure to retain such an electrostatic image that the potential of theunexposed portion not having been subjected to the image exposure duringthe secondary charging operation exhibits a positive value; applying atoner charged in an opposite polarity to that electrostatic image by adevelopment; and subsequently repeating a charge elimination (which maybe omitted) and an electrostatic image retention and development similarto the aforementioned ones. On the other hand, FIG. 17 shows theembodiment comprising the steps of: charging the image retainer 1negative by a primary charging operation contrary to the fact that theimage retainer 1 is charged positive in the embodiment of FIG. 16;charging the image retainer 1 positive by a subsequent second chargingoperation and a simultaneous image exposure; subjecting the wholecharged surface to an exposure so that the potential of the unexposedportion other than the portion having been subjected the image exposureduring the secondary charging operation is negative like the primarycharging operation; applying a toner in an opposite polarity by adevelopment to the electrostatic image at a portion which has beencharged positive by the image exposure during the secondary chargingoperation; .[.nad.]. .Iadd.and .Iaddend.subsequently repeating a chargeelimination (which may be omitted), and an electrostatic image retentionand a development like the aforementioned ones.

Incidentally, all FIGS. 12 to 17 show the step, at which the seconddevelopment has been conducted, and the embodiments in which the secondimage exposure is shifted from the first one. However, not only in theembodiments of FIGS. 14 to 17, in which the charge elimination and theprimary and the secondary charging operations are repeated at each time,but also in the embodiments of FIGS. 12 and 13, the subsequent imageexposing ray is incident upon the previous image exposed position, inwhich the electrostatic image is formed, by means for increasing thequantity of light of the image exposing ray the more for a later time sothat the toner of a color different from the previous one can be appliedin a superposed manner by the developments. Indicated at referenceletters T and T' in FIGS. 12 to 17 are toners in different colors, whichare applied to the surface of the image retainer.

In any of the embodiments of FIGS. 12 to 17, moreover, each developmentis conducted with the developer in a polarity opposite to that of theelectrostatic image thereby to provide an excellent effect that aremarkably clear color image can be reproduced with little scattering ofthe developer, and the reproducing apparatus is so constructed as isshown in FIG. 9 thereby to provide an advantage that the primary andsecondary chargers and the image exposing ray producing apparatus takingpart in the respective retentions of the electrostatic images can becommonly used. These embodiments of FIGS. 12 to 17 will be describedmore specifically in the following in connection with Examples 5 to 10.

EXAMPLE 5 (I.E., EMBODIMENT OF FIG. 12)

The reproducing apparatus shown in FIG. 9 was used. Despite of thisfact, the image retainer 1 was prepared by forming a transparentinsulating film having a thickness of 20 μm on a CdS photosensitivelayer having a thickness of 30 μm and had a circumferential speed of 180mm/sec. The image retainer 12 thus prepared was so charged by means of ad.c. scorotron corona discharger 2b, while being subjected to a uniformexposure by the lamp 2a of the primary charger 2, that its surfacepotential took a level of +1,000 V. Next, the image retainer 1 wascharged to have a surface potential of -100 V by means of the secondarycharger 3 composed of the scorotron corona discharger having an a.c.component. This charged surface was subjected to a writing exposure witha density of 16 dots/mm by means of the laser beam scanner using theHe-Ne laser, as shown in FIG. 2, to retain an electrostatic image havinga background potential of -100 V and an exposed portion potential of +50V. This electrostatic image was developed by the developing means 5shown in FIG. 3.

The developing means 5 used the developer, which was composed of: acarrier prepared by dispersing and containing 50 wt. % of magnetite in aresin to have an average particle diameter of 20 μm, a magnetization of30 emu/g and a resistivity of 10¹⁴ Ω cm or higher; and a non-magnetictoner prepared by adding 10 wt. parts of a benzine derivative as theyellow pigment and another charge controlling agent to the styrene-acrylresin to have an average particle diameter of 10 μm, under a conditionthat the ratio of the toner to the carrier was 20 wt. %. Moreover, thenon-contact jumping developing condition was resorted to, under whichthe developing sleeve 31 had an external diameter of 30 mm and a numberof revolutions of 100 r.p.m., under which the magnet 32 has its N and Smagnetic poles of a magnetic flux density of 900 gausses and a number ofrevolutions of 1,000 r.p.m., under which the layer of the developer inthe developed region had a thickness of 0.7 mm, under which the gapbetween the developing sleeve 31 and the image retainer 1 was 0.8 mm,and under which a superposed voltage containing a d.c. voltage of -50 Vand an a.c. voltage of 2.5 kHz and 2,000 V was applied to the developingsleeve 31.

While the developing image was being developed by the developing means5, the remaining developing means 6 to 8, as shown in FIG. 3, were heldin their undeveloping state. This was achieved by disconnecting thedeveloping sleeve 31 from the power supply 39 into its floating state,by grounding the same to the earth, or by positively applying the d.c.bias voltage, which had the same polarity as the charge of the imageretainer 1 but the opposite polarity to the charge of the toner, to thedeveloping sleeve 31. Of these, it is preferred to apply the d.c. biasvoltage. Since the developing means 6 to 8 were made to conduct theirdevelopments under the non-contact jumping developing condition like thedeveloping means 5, it was not necessary to eliminate the layer of thedeveloper layer from the developing sleeve 31. Of those developing means6 to 8: the developing means 6 used a developer which was prepared byreplacing the toner of the developer of the developing means 5 by atoner containing polytungstate as the .[.Magenta.]. .Iadd.magenta.Iaddend.pigment in place of the yellow pigment; the developer 7 used adeveloper which was prepared by replacing the same toner by a tonercontaining copper phthalocyanine as the cyan pigment; and the developingmeans 8 used a developer which was prepared by replacing the same toneby a toner containing carbon black as the black pigment. It is quitenatural that the toner containing other pigment and dye could be used asthe color toner, and that the order of the colors to be developed andthe order of the developing means could be suitably selected. Especiallyin case the positions of the image exposing rays were superposed, theorder of the colors to be developed had to be carefully determinedbecause it had a serial relationship with the clearness of the colorimage.

When the surface of the image retainer 1 developed by the developingmeans 5 arrived again at the position, where it was to be exposed to theimage exposing ray 4, after it had passed through the positions of thecorona discharger 15 and the pre-transfer lamp 10, both of which werenot necessary until the final transfer was conducted, the chargeeliminating means 13 and the cleaning means 14, both of which are heldin their inoperative states, and the primary charger 2 and the secondarycharger 3' which came into their paused states after having conductedthe primary and secondary charging operations, a second writingoperation was conducted with a shift of the dot positions, in a doubledlight quantity and in the same dot density by means of the same laserbeam scanner as the previous one. The electrostatic image thus obtainedhad a potential of +200 V for the background portion potential of -100V. The resultant electrostatic image was developed by the developingmeans 6 under the same conditions as those of the developing means 5except that a voltage having a d.c. component of 100 V and an a.c.component of 2.5 kHz and 2,000 V was applied to the developing sleeve31.

Likewise, at the third rotation of the image retainer 1, a writingoperation was conducted in a tripled light quantity by means of thelaser beam scanner to retain an electrostatic image having a potentialof +350 V for a background portion potential of -100 V. Thiselectrostatic iamge was developed by the developing means 7 under thesame conditions as those of the developing means 5 except that a voltagehaving a d.c. component of 250 V and an a.c. component of 2.5 kHz and2,000 V was applied to the developing sleeve 31. Likewise, moreover, awriting operation was conducted at the fourth rotation of the imageretainer 1 in a quadrupled light quantity by means of the laser beamscanner to retain an electrostatic iamge of +500 V for a backgroundpotential of -100 V. This electrostatic image was developed by thedeveloping means 8 under the same conditions as those of the developingmeans 5 except that a voltage having a d.c. component of 400 V and ana.c. component of 2.5 kHz and 2,000 V was applied to be developingsleeve 31.

At the stage when that fourth development was conducted so that thecolor image of four colors was retained on the image retainer 1, thecorona discharger 15 and the pre-transfer lamp 10 were operated to makethe color image liable to be transferred, and this color image wastransferred to the recording member P by the transfer means 11 and fixedby the fixing means 12.

The image retainer 1 having the color image thus transferred thereto hadits charges eliminated by the charge eliminating means 13 and itssurface cleared of the residual toners by the abutment against thecleaning blade of the cleaning means 14. As a result, the one colorimage reproducing cycle was completely finished at the time the surfacehaving the color image retained therein passed over the cleaning means14.

The color image thus reproduced was freed from any color mixing not onlyat the portion, to which the respective color toners were coarselyapplied, but also as the portion, to which the same were denselyapplied, so that it was remarkably clear.

EXAMPLE 6 (I.E., EMBODIMENT OF FIG. 13)

The same reproducing apparatus as that of the Example 5 was used, andprimary and secondary charging operations were conducted under the sameconditions as those of the Example 5 by means of the primary charger 2and the secondary charger 3'. After that, the writing operations wasconducted by the same laser beam scanner as that of the Example 5 but ina light quantity four times as large as that of the first writingoperation of the Example 5 and with the same dot density as that of theExample 5 to retain an electrostatic image having an exposed portionpotential of +500 V for a background portion potential of -100 V in theimage retainer 1 under the same conditions as those of the Example 5.

This electrostatic image was developed by the developing means 5 underthe same conditions as those of the Embodiment 5 except that asuperposed voltage composed of a d.c. voltage of 50 V and an a.c.voltage of 1.5 kHz and 1,000 V was applied to the developing sleeve 31.

Moreover, when the surface of the image retainer 1 thus developed by thedeveloping means 5 arrived again at the position of the secondarycharger 3', the image retainer 1 is so charged again by the secondarycharger 3' that its surface potential took a level of -100 V. Thatsurface was subjected to second writing and exposing operations underthe same conditions as the previous ones except the shift of the dotpositions by means of the same laser beam scanner to retain again anelectrostatic image. This electrostatic image was developed by thedeveloping means 6 using the same developer as that of the Example 5 andunder basically the same conditions as the developing conditions of thedeveloping means 5.

Like the second operation, moreover, the charging and exposingoperations were repeated to develop a third electrostatic image by thedeveloping means 7 and to develop a fourth electrostatic image by thedeveloping means 8 in a similar manner. In this case, each developmentwas conducted by suitably changing the d.c. bias component and theamplitude, frequency, duty ratio and so on of the a.c. component of thevoltage, which was to be applied to the developing sleeve 31, inaccordance with the changes, developing characteristics and colorreproducing state of the surface potential of the image retainer 1.

After the fourth development has been conducted to retain a color imageof four colors on the image retainer 1, the operations of reproducingthe color image was completed like the Example 5.

The reproduced image thus obtained was remarkably clear like that of theExample 5.

EXAMPLE 7 (I.E., EMBODIMENT OF FIG. 14)

The reproducing apparatus shown in FIG. 9 was used. Despite of thisfact, the image retainer 1 was prepared by forming a transparentinsulating film having a thickness of 10 μm on an α-Si photosensitivelayer having a thickness of 10 μm and had a circumferential speed of 180mm/sec. The image retainer 1 thus prepared was charged with a voltage of+700 V by the primary charger 2, while being uniformly exposed to thelamp 2a like the Example 5, and was then charged with a voltage of 0 Vby the secondary charger 3'. This charged surface was subjected to awriting operation by the laser beam scanner like the Example 5. Theelectrostatic image thus obtained had a potential of +300 V with respectto the background portion potential of 0 V. The resultant electrostaticimage was developed by the developing means 5 under the same conditionsas those of the Example 5 except that a voltage composed of a d.c.component of 100 V and an a.c. component of 500 Hz and 400 V was appliedto the developing sleeve. A second development was conducted by thedeveloping means 6 using the same developer as that of the Example 5 andunder the same conditions as those of the developing means 5 byexecuting the primary and secondary charging operations and the imageexposure under the same conditions as those of the first developmentafter the charge eliminating means 13 had been operated to eliminate thecharges (although this charge eliminating step might be omitted). Thirdand fourth developments were repeated in the same manner as that of thesecond one by means of the developing means 7 and 8, respectively. Thedevelopers of the developing means 7 and 8 were the same as those of theExample 5, respectively. Although substantially unnecessary forpractical purposes, however, the change of the conditions of the voltageto be applied to the developing sleeve was conducted like the Example 6in accordance with the potential change, the developing characteristicsand the color reproductivity of the image retainer 1. Thus, the colorimage reproducing operations were completed like the Example 5.

The reproducing image thus obtained was remarkably clear like theExample 5.

EXAMPLE 8 (I.E., EMBODIMENT OF FIG. 15)

The reproducing apparatus shown in FIG. 9 was used. Despite of thisfact, the image retainer 1 was prepared by forming a transparentinsulating film having a thickness of 10 μm on an α-Si photosensitivelayer having a thickness of 10 μm and had a circumferential speed of 180mm/sec. The image retainer 1 thus prepared was charged with a voltage of-700 V by the primary charger 2, while being uniformly exposed to thelamp 2a like the Example 5, and was then charged with a voltage of +300V by the secondary charger 3'. This charged surface was subjected to aslit exposure through the blue filter of an ordinary color reproducingmachine. As a result, there was obtained at a potential of +300 V anelectrostatic image which corresponds to the unexposed portion withrespect to the background portion of 50 V corresponding to the exposedportion. The resultant electrostatic image was developed by thedeveloping means 5 under the same conditions as those of the Example 5except that a voltage composed of a d.c. component of 50 V and an a.c.component of 500 Hz and 400 V was applied to the developing sleeve. Asecond development was conducted by the developing means 6 using thesame developer as that of the Example 5 and under the same conditions asthose of the developing means 5 by executing the primary and secondarycharging operations and the image exposure through a green filter underthe same conditions as those of the first development after the chargeeliminating means 13 had been operated to eliminate the charges(although this charge eliminating step might be omitted). Third andfourth developments were repeated in the same manner as that of thesecond one but by changing the exposing filters into a red filter and anneutral filter, respectively, to form an electrostatic image, and thiselectrostatic image was developed by means of the developing means 7 and8, respectively. The developers of the developing means 7 and 8 were thesame as those of the Example 5, respectively. Although substantiallyunnecessary for practical purposes, however, the change of theconditions of the voltage to be applied to the developing sleeve wasconducted like the Example 6 in accordance with the potential change,the developing characteristics and the color reproductivity of the imageretainer 1. Thus, the color image reproducing operations were completedlike the Example 5.

The reproduced image thus obtained was remarkably clear like the Example5.

EXAMPLE 9 (I.E., EMBODIMENT OF FIG. 16)

The reproducing apparatus shown in FIG. 9 was used. Despite of thisfact, the image retainer 1 was prepared by forming a transparentinsulating film having a thickness of 10 μm on an α-Si photosensitivelayer having a thickness of 10 μm and had a circumferential speed of 180mm/sec. The image retainer 1 thus prepared was charged with a voltage of+700 V by the primary charger 2, while being uniformly exposed to thelamp 2a like the Example 5, and was then charged with a voltage of -100V by the secondary charger 3'. This charged surface was simultaneouslysubjected to a slit exposure through the blue filter of an ordinarycolor reproducing machine, and then to a uniform exposure. As a result,there was retained at a potential of +300 V an electrostatic image whichcorresponds to the unexposed portion with respect to the backgroundportion of -100 V corresponding to the exposed portion. The resultantelectrostatic image was developed by the developing means 5 under thesame conditions as those of the Example 5 except that a voltage composedof a d.c. component of 0 V and an a.c. component of 500 Hz and 400 V wasapplied to the developing sleeve. A second development was conducted bythe developing means 6 using the same developer as that of the Example 5and under the same conditions as those of the developing means 5 byexecuting the primary and secondary charging operations, the imageexposure through a green filter, and the uniform exposure under the sameconditions as those of the first development after the chargeeliminating means 13 has been operated to eliminate the charges(although this charge eliminating step might be omitted). Third andfourth developments were repeated in the same manner as that of thesecond one but by changing the exposing filters into a red filter and anneutral filter, respectively, to form an electrostatic image, and thiselectrostatic image was developed by means of the developing means 7 and8, respectively. The developers in the developing means 7 and 8 were thesame as those of the Example 5, respectively. Although substantiallyunnecessary for practical purposes, however, the change of theconditions of the voltage to be applied to the developing sleeve wasconducted like the Example 6 in accordance with the potential change,the developing characteristics and the color reproductivity of the imageretainer 1. Thus, the color image reproducing operations were completedlike the Example 5.

The reproduced image thus obtained was remarkably clear like the Example5.

EXAMPLE 10 (I.E., EMBODIMENT OF FIG. 17)

The reproducing apparatus shown in FIG. 9 was used. Despite of thisfact, the image retainer 1 was prepared by forming a transparentinsulating film having a thickness of 10 μm on an α-Si photosensitivelayer having a thickness of 10 μm and had a circumferential speed of 180mm/sec. The image retainer 1 thus prepared was charged with a voltage of-700 V by the primary charger 2, while being uniformly exposed to thelamp 2a like the Example 5, and was then charged with a voltage of +300V by the secondary charger 3'. Simultaneously with this secondarycharging operation, the charged surface was subjected to a writingoperation by means of the laser beam scanner and then to a uniformexposure. As a result, there was retained at a potential of +300 V anelectrostatic image which corresponds to the exposed portion withrespect to the background portion of -100 V corresponding to theunexposed portion. The resultant electrostatic image was developed bythe developing means 5 under the same conditions as those of the Example5 except that a voltage composed of a d.c. component of 50 V and an a.c.component of 500 Hz and 400 V was applied to the developing sleeve. Asecond development was conducted by the developing means 6 using thesame developer as that of the Example 5 and under the same conditions asthose of the developing means 5 by executing the primary and secondarycharging operations, the image exposure, and the uniform exposure underthe same conditions as those of the first development after the chargeeliminating means 13 had been operated to eliminate the charges. Thirdand fourth developments were repeated in the same manner as that of thesecond one, and this electrostatic image was developed by means of thedeveloping means 7 and 8, respectively. The developers in the developingmeans 7 and 8 were the same as those of the Example 6, respectively.Although substantially unnecessary for practical purposes, however, thechange of the conditions of the voltage to be applied to the developingsleeve was conducted like the Example 6 in accordance with the potentialchange, the developing characteristics and the color reproductivity ofthe image retainer 1. Thus, the color image reproducing operations werecompleted like the Example 5.

The reproduced image thus obtained was remarkably clear like the Example5.

According to the Examples of the present invention thus far described,it is possible to change the potentials and polarities of theelectrostatic images in dependence upon the relative strengths of theprimary and secondary charging operations and to easily conduct thepositive or negative exposure. It is also possible to superpose thedevelopers under the non-contact jumping developing conditions andespecially to make the electrostatic images and the developers haveopposite polarities. As a result, the developers are easily applied tothe electrostatic images so that the adjustment of the developingconditions of the image retainer for the changes in the potential can befacilitated to reproduce a color image having a sufficient density andan excellent clearness while having little scattering of the developers.Since the common charger and image exposing ray producing means are usedfor each retention of the electrostatic images, moreover, there can beattained an effect that the recording apparatus can be reduced in sizeand cost without being troubled by registration.

Incidentally, the present invention can be applied to such an imageretainer as is applied to a base, as in electrofax paper, and that thecolor image retained thereon is not transferred but fixed. In this case,the pre-transfer lamp, the transfer means, the cleaning means and so oncan be dispensed with. It is true but the pre-transfer lamp 10 and thecharge eliminating means 15 can be omitted in the case of the transfer,and this transfer may be conducted not only the corona transfer one butalso a bias roller one, an adhesion transfer, a direct pressure one oron using an intermediate transfer member, whereas the fixture should notbe limited to the heat roller.

On the other hand, it is a great advantage that the polarities of thepotentials of the latent images of the image portion and the non-imageportion can be reversed by the balance in the strength between theprimary and secondary charging operations. Even if the polarities arecommon, however, the development can naturally be conducted by changingthe developing bias conditions. As to the potentials at this time, therelationship, in which the zero potential in FIGS. 12 to 17 is shiftedup or down, holds as it is. A similar development can be conducted ifthe developing bias is accordingly changed.

FIGS. 18 to 21 show other embodiments of the method of the presentinvention. Incidentally, FIGS. 18 to 21 all show the steps after thesecond development has already been conducted.

FIG. 18 shows the embodiment of the reproducing method of the presentinvention, comprising the steps of: uniformly subjecting the surface ofthe image retainer 1 in the initial state, which has had its chargeseliminated by the charge eliminating means 13 of the reproducingapparatus of FIG. 1 and has been cleaned by the cleaning means 14 tohave a zero potential, to a first charging operation by means of thecharger 2; subjecting the charged surface to a first image exposure ofdifferent color by the image exposing means 4, as shown in FIG. 2, toretain an electrostatic image in which the potential of the exposedportion PH is dropped with respect to the background potential of theunexposed portion DA; firstly developing the electrostatic image by anyone of the developing means 5 to 8, which uses as its developer thecolor toner corresponding to the first image exposure, so that thepotential of the electrostatic image by the first image exposure iselevated up to the background potential as a result that the color tonerhaving been frictionally charged with the same polarity as that of thecharges of the image retainer 1 is applied; secondly charging the imageretainer 1 uniformly again at the second rotation by means of thecharger 2 with or without the charge elimination by the chargeeliminating means 13, because that portion cannot retain theelectrostatic image even if it is subjected again to the image exposure;conducting a second image exposure by the same image exposing means 4 asthe previous one with the same spot density as that of the previousimage exposure and in a manner that the positions are superposed atleast partially; subsequently conducting a second development by anotherdevelopment means using as its developer the color toner correspondingto the second image exposure; subsequently repeating in a like mannerthird and fourth charging operations and image exposures to retain acolor image having a plurality of color toner images superposed on theimage retainer 1; operating one or both of the pre-transfer charger 9and the pre-transfer exposing lamp 10, if necessary, from the step atwhich the fourth development is conducted to the step as which the colorimage finishes its passage; transferring the color image by the transfermeans 11 to the recording member P being fed in synchronism with therotations of the image retainer 1; fixing the color image transferred tothe recording member P by the fixing means 12; eliminating the chargesfrom the surface of the image retainer 1, from which the color image hasbeen transferred, by the charge eliminating means 13; and cleaning theimage retainer 1 by the cleaning means 14 until the initial state isrestored, thus completing the one cycle of the color image reproduction.Thus, by conducting the image exposure of each time such that the spotpositions are superposed at least partially, it is possible to preventthe color image retained from having its picture element density droppedso that the color image can be reproduced densely and finely in itscolors. Incidentally, the retention of such an electrostatic image bythe second image exposure at a position different from that havingpreviously trapped the toner T as has a lower potential at its exposedportion PH that the background potential at its unexposed portion DA isintended to shown the case in which the color image has in thatparticular position none of the picture elements of the color of thetoner T. Indicated at reference letter T' is the toner which has beenapplied at the second development. Although FIG. 18 shows the case inwhich the spot positions of the image exposures of the respective timesare completely superposed, the spot positions may be partiallysuperposed. In the method of the present invention, moreover, since theretentions of the electrostatic images at the respective times can beconducted by the common apparatus, as has been described hereinbefore,the reproducing apparatus can be conducted in a small size and at a lowcost, and the synchronous control of the image exposure can be easilyconducted.

FIG. 19 shows the embodiment of the present invention, which is the sameas the embodiment of FIG. 18 until the first development but differenttherefrom in that, after the first development, the surface of the imageretainer 1 is either secondly charged by the charger, after it has beenuniformly exposed by the pre-transfer exposing lamp 10 or the chargeeliminating lamp of the charge eliminating means 13, or uniformly butweakly exposed by means of the exposing lamp 3 after it has beensecondly charged by the charger 2, so that the electrostatic imageretainer portion having trapped the previous toner T is made to havesuch a slightly higher potential than that of the background portionthat it is liable to retain the electrostatic image, whereupon, like theembodiment of FIG. 18, the second image exposure and the seconddevelopment are executed, followed by repeating the third and fourthimage exposures and developments in a similar manner to retain the colorimage. The embodiment of FIG. 19 is suitable in case it is undesirableto apply the subsequent toner T' to the previous toner T. Generallyspeaking, more specifically, most of the images are required to have areproductivity of black letters. According to the method beingdescribed, the light is not passed fully at the subsequent writing andthe color toner to be subsequently applied in a superposed manner can beprevented from being applied by firstly conducting the writing operationof a black letter portion so that the vagueness of colors and the shiftof positions can be prevented. Thus, it is possible to obtain an imagein which a preferential color is stressed. In combination with theembodiment of FIG. 18, on the other hand, an image of the selected colorcan be stressed or weakened. It is quite natural that the chargeeliminating step and the charging step can be introduced after the firstdevelopment and after the subsequent process is entered.

FIGS. 20 and 21 show the embodiments of the present invention, which areimproved in the problem of mixing by applying the subsequent toner T' ina manner to surround the previous toner T, because the mixing state of acolor toner is changed in dependence upon the fixing method or the colorsuperposing order to change the color balance if the subsequent toner T'is superposed upon the previous toner T. The embodiments of FIGS. 20 and21 correspond to those of FIGS. 18 and 19, respectively, but aredifferent therefrom in that the spot of the second image exposing ray,i.e, the subsequent image exposing ray is made so large by the imageexposing means 4, i.e., by means of the lens 27 of the laser beamscanner of FIG. 2 as to enclose the spot of the first image exposingray, i.e., the previous image exposing ray. Incidentally, the boundariesbetween the unexposed portion DA and the exposed portion pH in thesecond image exposure are shown in two ways, i.e., the upper portionshows the spot of the second image exposing ray, and the lower portionshows the exposing area shield by the center toner image. In theembodiments of FIGS. 20 and 21, too, the color image retained can beprevented from having its picture element density dropped so that it canhave its colors retained densely and finely, and the problem of tonermixing is eliminated so that a color image having clearer colors can beretained in the embodiments of FIGS. 20 and 21. Especially in theembodiment of FIG. 21, as compared with the embodiment of FIG. 20, theportion, in which the previous spot and the subsequent spot aresuperposed, does not trap or is reluctant to trap the subsequent tonerT' on the previous toner T thereby to prevent the toners T and T' frombeing mixed to exhibit a mixed color at the portion having the spotssuperposed because the subsequent toner T' is applied only around theprevious spot, so that a beautiful blended color can be attained.

Incidentally, in the embodiments of FIGS. 18 and 19, the toners aremixed so that the color developing sequence exerts great influence uponthe clearness of the color image. It is, therefore, important todetermine the sequence of the developments. In the embodiments of FIGS.20 and 21, however, the importance of the developing order is not sohigh as that of the embodiments of FIGS. 18 and 19. Despite of thisfact, how the colors are arranged from the central portion to the outerside still influences upon the tone of the color image.

The foregoing are the embodiments in which all the developments by thedeveloping means 5 to 8 are conducted by the developing method ofapplying the toner charged with the same polarity as that of thebackground potential to the electrostatic image corresponding to theexposed portion having a lower potential than that of the backgroundpotential. According to the reproducing apparatus shown in FIG. 9,however, the developments of the method of the present invention can beconducted by the developing method of applying the toner charged withthe polarity opposite to that of the electrostatic image to theelectrostatic image.

FIG. 22 shows the embodiment of the reproducing method of the presentinvention, comprising the steps of: subjecting like the above embodimentthe surface of the image retainer 1, which has had its chargeseliminated by the charge eliminating means 13 of the reproducingapparatus of FIG. 9 and has been cleaned by the cleaning means 14 tohave a zero potential, to a primary charging operation by means of thecharger 2; subjecting the surface to a secondary charging operation bymeans of the secondary charger 3'; subsequently subjecting the chargedsurface to a first image exposure by the image exposing means 4 likethat of FIG. 2; firstly developing the electrostatic image, in which theabsolute value of the potential of the exposed portion pH retained ishigher than the background potential of the unexposed portion DA, bythat one of the developing means 5 to 8, which uses as its developer thetoner for effecting the changing operation with a polarity opposite tothat of the electrostatic image of the color corresponding to the firstimage exposure, so that the surface potential of the electrostatic imageretaining portion is dropped by the toner having the opposite polarityapplied; discharging again the surface of the image retainer 1 beforethe image exposure at the second rotation by means of the second charger3' to retain the electrostatic image in the previously developedelectrostatic image retaining portion, too; conducting again a secondimage exposure by the same image exposing means 4 as the previous onewith the same spot density as that of the previous image exposure and ina manner that the positions are superposed at least partially;subsequently conducting a development by another developing means usingas its developer the color toner corresponding to the second imageexposure having the same charging characteristics as those of the firstdevelopment; subsequently repeating in a like manner third and fourthelectrostatic image retentions and developments, thus completing the onecycle of the color image reproduction like the embodiments of FIGS. 18to 21 after the color image has been retained. According to thisembodiment, all the developments are conducted by applying the tonersfor charging the electrostatic images with an opposite polarity so thatthe control of the developing densities of the respective colors iseasier than the embodiments of FIGS. 18 to 21.

FIG. 23 shows an embodiment of the present invention, which is differentfrom the embodiment of FIG. 22 in that not only the secondary chargingoperation by the secondary charger 3' is conducted but also the primarycharging operation is conducted beforehand by the primary charger 2during the time period between the first development and the secondimage exposure, and in that the charge elimination is also conducted bythe charge eliminating means 13 prior to the primary charging operation.According to the embodiment of FIG. 23, it becomes possible to apply thetoner T' in the same density to the toner T having been previouslyapplied.

FIGS. 24 and 25 show embodiments of the method of the present invention,which are different from the embodiments of FIGS. 22 and 23 in that thesecond image exposure is conducted in a manner to change the spotdiameter like the embodiments of FIGS. 20 and 21. According to theembodiments of FIGS. 24 and 25, there can be attained a result that thevagueness due to the color mixing can be eliminated like the embodimentsof FIGS. 10 and 21.

Next, the embodiments of FIGS. 18 to 21 and FIGS. 22 to 25 thus fardescribed will be described in more detail in the following as Examples11 to 18, respectively.

EXAMPLE 11

The reproducing apparatus of FIG. 1 was used. However, the exposing lamp3 was not used, but the image retainer 1 had a photosensitive surfacelayer of Se and a circumferential speed of 180 mm/sec. This imageretainer 1 has its surface charged to +500 V by means of the charger 2using the scorotron corona discharger, and the charged surface wassubjected to a first image exposure in a density of 16 spots/mm by meansof the laser beam scanner of FIG. 2 using the He-Ne laser.

As a result, there was retained in the image retainer 1 an electrostaticimage which had a background potential of +500 V but an exposed portionpotential of +30 V. The resultant electrostatic image was subjected to afirst development by the developing means 5 shown in FIG. 3.

The developing means 5 used the developer, which was composed of: acarrier prepared by dispersing and containing 50 wt. % of magnetite in aresin to have an average particle diameter of 30 μm, a magnetization of30 emu/g and a resistivity of 10¹⁴ Ω cm or higher; and a non-magnetictoner prepared by adding 10 wt. parts of a benzine derivative as theyellow pigment and another charge controlling agent to the styrene-acrylresin to have an average particle diameter of 10 μm, under a conditionthat the ratio of the toner to the carrier was 20 wt. %. Moreover, thenon-contact jumping developing condition was resorted to, under whichthe developing sleeve 31 had an external diameter of 30 mm and a numberof revolutions of 100 r.p.m., under which the magnet 32 has its N and Smagnetic poles of a magnetic flux density of 1,000 gausses and a numberof revolutions of 1,000 r.p.m., under which the layer of the developerin the developed region had a thickness of 0.7 mm, under which the gapbetween the developing sleeve 31 and the image retainer 1 was 0.8 mm,and under which a superposed voltage containing a d.c. voltage of +400 Vand an a.c. voltage of 1.5 kHz and 1,000 V was applied to the developingsleeve 31.

While the developing image was being developed by the developing means5, the remaining developing means 6 to 8, as shown in FIG. 3, were heldin their undeveloping state. This was achieved by disconnecting thedeveloping sleeve 31 from the power supply 39 into its floating state,by grounding the same to the earth, or by positively applying the d.c.bias voltage, which had the polarity opposite to that of the charge ofthe image retainer 1 i.e., the opposite polarity to the charge of thetoner, to the developing sleeve 31. Of these, it is preferred to applythe d.c. bias voltage. Since the developing means 6 to 8 were made toconduct their developments under the non-contact jumping developingcondition like the developing means 5 it was not necessary to especiallyeliminate the layer of the developer from the developing sleeve 31. Ofthose developing means 6 to 8: the developing means 6 used a developerwhich was prepared by replacing the toner of the developer of thedeveloping means 5 by a toner containing polytungstate as the.[.Magenta.]. .Iadd.magenta .Iaddend.pigment in place of the yellowpigment; the developer 7 used a developer which was prepared byreplacing the same toner by a toner containing copper phthalocyanine asthe cyan pigment; and the developing means 8 used a developer which wasprepared by replacing the same toner by a toner containing carbon blackas the black pigment. It is quite natural that a toner containing otherpigment and dye could be used as the color toner, and that, as has beentouched hereinbefore, the sequence of the colors to be developed andaccordingly the sequence of the developing means could be suitablyselected.

The surface of the image retainer 1 thus having been subjected to thefirst development was subjected, after it had been secondly charged with+500 V at the second rotation by means of the charger 2 while thepre-transfer exposing lamp 10 being operated but the charge eliminatingmeans 13 and the cleaning means 14 being left inoperative, to a secondimage exposure again in the superposed spot positions and in the samespot density by means of the same laser beam scanner and then to asecond development using the .[.Magenta.]. .Iadd.magenta .Iaddend.tonerby the developing means 6. Likewise, a third development using the cyantoner by the developing means 7 and a fourth development using the blacktoner by the development means 8 were repeated. In each of thedevelopments, incidentally, the developing density of each color can beadjusted in accordance with the changes of the surface potential of theimage retainer 1, the developing characteristics, the colorreproductivity and so on by changing the d.c. bias components and theamplitude and frequency of the voltage to be applied to the developingsleeve 31, and the selecting time of the time selecting conversion.

After the fourth development was conducted so that the four-color imagewas retained on the image retainer 1, it was made liable to betransferred by the pre-transfer charger 9 and the pre-transfer exposinglamp 10 so that it was transferred to the recording member P by thetransfer means 11 until it was fixed by the fixing means 12. The imageretainer 1 having the color image transferred thereto had its chargeseliminated by the charge eliminating means 13 and its surface cleared ofthe residual toners by its abutment against the cleaning blade or furbrush of the cleaning means 14. The one cycle of the color imagereproduction was completely ended at the time when the surface havingretained the color image therein passes over the cleaning means 14.

The color image thus reproduced had the vagueness in color due to thecolor mixing but had a high density of spot picture elements and afinely expressed pattern.

EXAMPLE 12

The same reproducing apparatus of FIG. 1 as that of the Example 11 wasused. In this case, however, the apparatus is equipped with the exposinglamp 3. And, a first development was conducted under absolutely the sameconditions as those of the Example 11 except that a superposed voltageof a d.c. voltage of +40 V and an a.c. voltage of 2 kHz and 1,000 V wasapplied in the development to the developing sleeve 31. Next, at thesecond rotation, the surface of the image retainer 1 having beensubjected to the first development was secondly charged with +600 V bymeans of the charging means 2 and was then subjected to a uniformly andweak exposure by the exposing means 3 to take a surface potential of+500 V. As a result, the surface potential of the portion having trappedthe toner T by the first development came into a slightly higher statethan +500 V. Therefore, this image retainer 1 had its surface subjectedto a second image exposure and a second development like the Example 11.The exposure, uniform and weak exposure and development described abovewere repeated thirdly and fourthly thereby to conduct the color imagereproduction like the Example 11.

The color image thus reproduced was finely expressed not differently ofthat of the Example 11 except that the tone of the mixed-color portionis stressed slightly better in its previously color than that of theExample 11.

Incidentally, in this Example, too, similar effects can be attained evenif the charging operation is conducted by means of the charger 2 after auniform exposure using the pre-transfer exposing lamp 10 or the exposinglamp of the charge eliminating means 13 in place of the chargingoperation and the uniform and weak exposure.

EXAMPLE 13

The same reproducing apparatus as that of the Example 11 was used. Thereproduction of a color image was conducted absolutely similar to theExample 11 except that a first image exposure using a spot having adiameter of 20 μm, a second image exposure using a spot having adiameter of 30 μm, a third image exposure using a spot having a diameterof 40 μm, and a fourth image exposure using a spot having a diameter of50 μm in the same spot position and in the same density of 16 spots/mmwere conducted by the switching operation of the lens 27 of the laserbeam scanner thereby to retain an electrostatic image having a potentialof +50 V with respect to the background potential of +600 V, that asuperposed voltage of a d.c. voltage of +450 V and an a.c. voltage of1.5 kHz and 1,000 V was applied for the development to the developingsleeve 31, and that the colors were superposed in the order of theblack, cyan, red and yellow toners.

The color image thus reproduced was substantially cleared of anyvagueness by the color mixing so that it has a fine and clear tone.

Incidentally, in this Example, an identical color image could beattained even if the charging operation between the first developmentand the second image exposure, i.e., the charging operation between theprevious development and the subsequent image exposure was omitted.

EXAMPLE 14

The same reproducing apparatus as that of the Example 12 was used. Thereproduction of a color image was conducted under absolutely the sameconditions as those of the Example 12 except that the image exposureswere conducted in the same manner as the Example 13 to retain the sameelectrostatic image, and that a superposed voltage of a d.c. voltage of30 .[.450.]. V and an a.c. voltage of 2 kHz and 500 V was applied forthe development to the developing sleeve 31.

The color image thus reproduced had a fine and clear color tone whichwas hardly different from that of the Example 13.

EXAMPLE 15

The reproducing apparatus of FIG. 9 was used. The image retainer 1 wasprepared by laying a transparent insulating surface layer having athickness of 20 μm on a photosensitive layer of CdS having a thicknessof 30 μm, and had a circumferential speed of 180 m/sec. The imageretainer 1 thus prepared was primarily charged to have a surfacepotential of +1,000 V by means of the d.c. scorotron corona dischargerwhile being uniformly exposed by the exposing lamp of the primarycharger 2. Next, the image retainer 1 was charged to have a surfacepotential of -100 V by means of the secondary charger 3' which isconstructed of the scorotron corona discharger having an a.c. component.The resultant charged surface was subjected to a first image exposure ina density of 16 spots/mm by means of the laser beam scanner of FIG. 2using the He-Ne laser to retain an electrostatic image exhibiting apotential of +200 V with respect to the background potential of -100 V.The resultant electrostatic image was firstly developed by thedeveloping means 5 under the same conditions as those of the Example 11except that only an a.c. voltage component of 1.5 kHz and .[.1.000.]..Iadd.1,000 .Iaddend.V was applied to the developing sleeve 31, and thatthe charging polarity of the toner was opposite to that of theelectrostatic image. In this Example, moreover, a second image exposureand a second development were conducted like the Example 11 after asecondary charging operation by the secondary charger 3' was conductedagain at the second rotation of the image retainer 1. Likewise, asecondary charging operation, an image exposure and a development weresubsequently repeated thirdly and fourthly, and reproduction of thecolor image was then conducted like the Example 11.

Since, in this Example, the developments were effected by the coulombattractive force, the density adjustment of the color image reproducedcould be conducted more easily than the cases of the Examples 11 to 14,by which the toners for charging in the same polarity were applied tothe electrostatic image, so that the color image obtained had the samecolor tone as that by the Example 11.

EXAMPLE 16

The reproduction of a color image was conducted under the sameconditions as those of the Example 15 except that the image retainer 1was prepared by placing a transparent insulating surface layer having athickness of 10 μm on an α-Si photosensitive layer having a thickness of10 μm, that a primary charging operation was effected to +700 V by meansof the primary charger 2, that a secondary charging operation waseffected to 0 V by the secondary charger 3' to retain electrostaticimages, the first one of which had a potential of +300 V and the secondand later of which had a similar potential with respect to thebackground potential of 0 V, in the image exposures by the laser beamscanner, that the voltage to be applied to the developing sleeve 31 forthe development was a superposed one composed of a d.c. voltage of +100V and an a.c. one of 500 Hz and 400 V, and that a charge elimination bythe charge eliminating means 13, the primary charging operation by theprimary charger 2, and the secondary charging operation by the secondarycharger 3' were conducted prior to the second and later image exposures.

In this Example, the color tone of the color image reproduced wiresimilar to that of the Example 11, and the densities of the respectivecolors could be better adjusted.

EXAMPLE 17

The reproduction of the color image was conducted under absolutely thesame conditions as those of the Example 15 except that the imageexposure by the laser beam scanner was conducted, like the Example 13,firstly with a spot having a diameter of 20 μm, secondly with a spothaving a diameter of 30 μm, thirdly with a spot having a diameter of 40μm, and fourthly with a spot having a diameter of 50 μm, in the samespot position and in the same density of 16 spots/mm to retain anelectrostatic image having a potential of +400 V with respect to thebackground potential of -100 V, and that the colors were superposed inthe order of the black, cyan, red and yellow toners.

The color image thus reproduced had a color tone similar to that by theExample 13 but had a clearer tone.

In this Example, too, the density adjustments of the respective colorscould naturally be easily effected.

EXAMPLE 18

The reproduction of the color image was conducted under absolutely thesame conditions of those of the Example 16 except that the imageexposure by the laser beam scanner used the same spot and spot densityas those of the Example 17 to retain an electrostatic image having apotential of +300 V at each time with respect to the backgroundpotential of 0 V.

The color image thus reproduced was substantially the same as that bythe Example 17.

In this Example, too, the density adjustments of the respective colorscould naturally be easily effected.

According to the Examples of the present invention thus far described,it is possible to make the spot densities fine thereby to reproduce afine color image and to prevent any color mixing thereby to reproduce acolor image having a clear color tone. Since the retentions of theelectrostatic images are conducted by the common apparatus, moreover,there can be attained excellent effects that the reproducing apparatuscan be constructed in a small size and at a low cost, and that thesynchronous control of the image exposure as to the image retainer isfacilitated.

Incidentally, the present invention can be applied to the case, in whichthe image retainer has a belt or sheet shape, or to such an imageretainer, e.g., electrofax paper as is placed on a base as can fixwithout any transfer the color image retained thereon by the toners. Inthis case, it is highly necessary to consider the superposing sequenceof the color toners, but there arises an advantage that the pre-transferlamp, the transfer means and the cleaning means can be dispensed with.Despite of this fact, the pre-transfer lamp and the charge eliminatingmeans can be omitted in case the toners have predetermined polaritiesand quantities of charges so that they can be transferred. On the otherhand, the transfer may be not only the corona type but also a biasroller type, an adhesion type and a pressure type through anintermediate transfer member. It is quite natural that the fixingoperation should not be limited to a heat roller type.

The methods of the Examples 15 to 18 according to the present inventionare highly advantageous in that the polarities of the potentials at theimage portion and the non-image portion can be reversed by the balancebetween the strengths of the primary and secondary charging operations.However, the development can be effected even by using the samepolarities and by changing the developing bias conditions. As to thepotentials at this time, there holds as it is the relationship in whichthe zero potential of FIGS. 21 to 23 is shifted up and down. If thedeveloping bias is accordingly changed, a like development can be made.Moreover, those methods can naturally be applied even to the NP- orKIP-method.

FIGS. 26 and 27 are schematic views showing the constructions of theembodiments of the reproducing apparatus which are used for the methodof the present invention, respectively. FIGS. 28 to 30 are flow chartsfor the method of the present invention, respectively.

In FIG. 26, reference numeral 41 indicates a drum-shaped image retainerwhich is constructed by laying a dielectric layer such as a resin on ametal base and which is made rotatable in the direction of arrow, andnumeral 43 indicates an electrostatic recording head which is equippedwith needle discharge poles. The remaining portions are identical tothose of the example of FIG. 1.

The pre-transfer charger 9 may be omitted in case the transfer can besufficiently effected merely by the transfer means 11. The electrostaticrecording head 43 is used to form an electrostatic image having acharged spot distribution on the dielectric layer of the image retainer41 by means of the needle discharge poles which are arrayed in one orplural rows.

Of the toners: the black toner is similar to that of the two-componentdeveloper of the prior art; the cyan toner is prepared by adding copperphthalocyanine in place of carbon black having a black color; theMagenta is prepared by similarly adding polytungstophosphate; and theyellow toner is prepared by similarly adding a benzidine derivative.However, those toners should not be limited to those color toners madeof such pigments, but it is naturally possible to use color toners madeof dyes and to add an electrification controlling agent or the like, ifnecessary. On the other hand, the sequence of the colors to be developedby the developing means 5 to 8 using the developers of different colortoners has to be carefully determined because it exerts influences uponthe tone of the color image.

The method of the present invention can be practised by the reproducingapparatus of FIG. 26 described above but can also be carried out by thereproducing apparatus shown in FIG. 27.

The reproducing apparatus of FIG. 27 is one in which a series ofrecording members are formed with dielectric layers on their surfaces toprovide an image retainer 41'. The retentions and developments ofelectrostatic images are repeated while the image retainer 31' is beinglinearly conveyed. Along the conveyor passage of the image retainer 41',the prewriting charger 2, the electrostatic recording head 43 and thedeveloping means 5 to 8 are repeatedly arranged side by side, and thefixing means 12 for fixing the color image to the image retainer 41' isdisposed at the last position. The reproducing apparatus underconsideration does not require the pretransfer charger, the transfermeans, the charge eliminating means and the cleaning means but canreproduce a series of color images. In order that the image retainer 41'may not depend, however, it is necessary to strengthen the tension or toprovide such a supporting roller midway as to prevent the toners appliedto the image retainer 41' from being offset, although not shown.

In the reproducing apparatus shown in FIG. 26, too, the pre-transfercharger 9, the transfer means 11, the charge eliminating means 13 andthe cleaning means 14 can be dispensed with if the image retainer 41 isprepared by rolling an image retainer similar to the image retainer 41',which is used in the reproducing apparatus of FIG. 27, on a drum.

The method of the present invention, as is exemplified by theembodiments of FIGS. 28 to 30, can be practised by the reproducingapparatus thus far described. Incidentally, FIGS. 28 to 30 all show thesteps after a second development has been finished.

The embodiment of FIG. 28 shows the method of the present invention,comprising the steps of: subjecting the surface of the image retainer 41to a first writing operation by means of the electrostatic recordinghead 43, either from the initial state (which is shown to be a chargedstate), in which the surface of the image retainer 41 has its chargeseliminated by one or both of the charge eliminating means 13 and 13,cleaned by the cleaning means 14 and charged to be positive or negativeby the pre-writing charger 2, if necessary, according to the reproducingapparatus of FIG. 26 or from the initial state, in which the imageretainer 41' is conveyed from the left and charged to be negative orpositive by the first pre-writing charger 2, if necessary, according tothe reproducing apparatus of FIG. 27, thereby to retain an electrostaticimage at a potential having a polarity different from that of thebackground potential; firstly developing that electrostatic image by thedeveloping means 5; conducting a second writing operation by theelectrostatic recording head 43 after the uniform charging operation bythe charger 2, if necessary, either when the image retainer 41 comesinto its second rotation, according to the reproducing apparatus of FIG.26, or when the image retainer 41' advances to the position of the nextcharger, according to the reproducing apparatus of FIG. 27; secondlydeveloping the electrostatic image thus retained by the developing means6; .[.subsequtnely.]. .Iadd.subsequently .Iaddend.repeating third andfourth writing and developing operations in a similar manner so that acolor image having superposed color images is retained on the imageretainer 41 or 41'; and either fixing the resultant color image to therecording member P by means of the fixing means 12, after the colorimage has been made reluctant to be transferred by the pretransfercharger 9 so that it is transferred to the recording member P by thetransfer means 11, according to the reproducing apparatus of FIG. 26, ordirectly fixing the same color image to the image retainer 41' by thefixing means 12 according to the reproducing apparatus of FIG. 27.According to the reproducing apparatus of FIG. 26, moreover, the surfaceof the image retainer 41 thus having the color image transferred theretohas its charges eliminated by the charge eliminating means 13, andcleared of the residual toners by the cleaning means 14, and further hasits charge eliminated, if necessary, by the charge eliminating means 13,thus ending one cycle of the color image reproduction. According to thereproducing apparatus of FIG. 27, on the other hand, the portion of theimage retainer 41', which has been formed with the color image, ends itssteps of reproducing the color image when it completely passes thefixing means 12.

Moreover, the embodiment of FIG. 29 uses the reproducing apparatus ofFIG. 26 and is similar to that of FIG. 28 except that the image retainer41 having the toner images retained thereon has its charges eliminatedby the charge eliminating means 13 before a subsequent image retainingstage is entered after each development.

The embodiment of FIG. 30 resorts to the reproducing apparatus of FIG.26 and is different from that of FIG. 28 in that the pre-writing charger2 is operated before each writing operation.

Incidentally, reference letters T and T' appearing in FIGS. 28 to 30indicate toners of different colors, which are applied to the imageretainer 41 or 41'.

In the method of the present invention, the developing means other thanthat conducting each development under the non-contact jumpingdeveloping conditions can be easily held in an inoperative state, evenif the developer layer is not removed from the developing sleeve 31, bydisconnection of the developing sleeve 31 from the power supply 39 intoa floating state, by grounding the developing sleeve 31 to the earth, orpositively applying such a d.c. bias voltage to the developing sleeve 31as has a polarity identical to that of the electrostatic image, i.e.,opposite to that of the charges of the toners. Of these, the applicationof the bias voltage having the opposite polarity to that of the tonersmay be preferably used to hold the developing means in the inoperativestate.

Next, the embodiment of FIGS. 28 to 30, which are practised by thereproducing apparatus of FIG. 26, will be described in more detail asthe following Examples 19 to 21, respectively.

EXAMPLE 19

The reproducing apparatus shown in FIG. 26 was used. The image retainer41 was prepared by laying an insulating layer having a thickness of 20μm on an aluminum base and had a circumferential speed of 180 mm/sec.The image retainer 41 thus prepared had its surface charged to -100 V bymeans of the pre-writing charger 2 using the scorotron corona dischargerand then subjected to writing operation in a distribution density of 10spots/mm by means of the electrostatic recording head 43 the needleelectrodes of which had their tips spaced by about 30 μm from thesurface of the image retainer 41. As a result, there was retained on theimage retainer 41 an electrostatic image which had a written portionpotential of +200 V with respect to the background portion potential of-100 V. The resultant electrostatic image was firstly developed by thedeveloping means 6 shown in FIG. 3. This developing means 6 used thedeveloper, which was composed of: a carrier having 50 wt. % of magnetitedispersed and contained in a resin and having an average particlediameter of 20 μm, a magnetization of 30 emu/g and a resistivity of 10¹⁴Ω or higher; and a nonmagnetic toner prepared by adding 10 wt. % ofcopper phthalocyanine and another electrification control agent as thecyan pigment to the styrene-acryl resin and which had an averageparticle diameter of 10 μm, under the condition of the ratio of 10 wt. %of the toner to the carrier. Moreover, the non-contact jumpingdeveloping conditions were resorted to under which the developing sleeve31 had an external diameter of 30 mm and a number of revolutions of 100r.p.m., under which the magnet 32 had a magnetic flux density of its Nand S magnetic poles of 1,000 gausses and a number of revolutions of1,000 r.p.m., under which the developer layer had a thickness of 0.7 mmat its developed portion, under which the gap between the developingsleeve 31 and the image retainer 1 was 0.8 mm, and under which a biasvoltage having a d.c. voltage component of 0 V and an a.c. voltagecomponent of 1.5 kHz and 1,000 V was applied to the developing sleeve.

The surface of the image retainer 41 having been firstly developed wassubjected to such a second writing operation with a spot position shiftfrom the first writing operation but in the same spot density again bythe same electrostatic recording head 43 but without operating thepre-transfer charger 9, the charge eliminating means 13 and 13, thecleaning means 14 and the pre-writing charger 2 that the written portiontook a potential of +300 V. Next, a second development was conducted bythe developing means 6 under the same conditions as those of thedeveloping means 5 except that the toner of the developer used one whichwas prepared by adding polytungstophosphate as the Magenta pigment inplace of the cyan pigment, and that a bias voltage having a d.c. voltagecomponent of 100 V and an a.c. voltage component of 1.5 kHz and 1,000 Vwas applied. Likewise, a third writing operation for elevating thepotential of the written portion to +400 V and a third development wereconducted by the developing means 7 under the same conditions as thoseof the developing means 5 except that the toner of the developer usedone which was prepared by adding a benzidine derivative as the yellowpigment, and that the developing bias was composed of a d.c. componentof 200 V and an a.c. component of 1.5 kHz and 1,000 V. Moreover, afourth writing operation for raising the potential of the writtenportion to +500 V and a fourth development were conducted by thedeveloping means 8 under the same conditions as those of the developingmeans 5 except that the toner of the developer used one which wasprepared by adding carbon black as the black pigment, and that thedeveloping bias had a d.c. component of 300 V and an a.c. component of1.5 kHz and 600 V. The color image thus retained on the image retainer41 was transferred to and fixed on the recording member P. Moreover, thesurface of the image retainer 41 thus having the color image transferredthereto had its charges eliminated by the charge eliminating means 13and cleared of the residual toners by the cleaning means 14.

The reproduced image thus obtained had little mixing of the color tonersand was a remarkably clear color image.

Incidentally, in this Example, the spot position of the subsequentwriting operation may be superposed of that of the previous writingoperation, or, the discharge voltage of the electrostatic recording head43, and the voltage value, frequency and selected time of the d.c. ora.c. component of the volatage to be applied to the developing sleevemay be so changed in the writing and/or developing operations as toadjust the developed densities of the respective colors. If the spotpositions of the writing operations are superposed, the color mixingoccurs to make the colors liable to be vague. However, the tone can beenhanced by increasing the spot density. In this case, moreover,especially the sequence of colors to be developed plays an importantrole. By adjusting the developed densities of the respective colors inthe manner thus far described, moreover, it is possible to obtain acolor image which has a changed tone.

EXAMPLE 20

The color image reproduction was conducted by the use of the samereproducing apparatus as that of the Example 19 and under the sameconditions as those of the Example 19 except that the charging operationof the Example 19 by the pre-writing charger 2 prior to the firstwriting operation was not conducted to form an electrostatic imagehaving a potential of +150 V with respect to the background potential of0 V by a first writing operation, that a superposed voltage having ad.c. voltage of +50 V and an a.c. voltage of 3 kHz and 2,000 V wasapplied as the bias voltage upon the development to the developingsleeve 31, and that charge elimination was conducted before second andlater writing operations by the charge eliminating means 13 to retain anelectrostatic image having a potential of +150 V with respect to thebackground potential of 0 V even in the second and later writingoperations. The reproduced image thus obtained was a color image havingan excellent clearness like that of the Example 19.

EXAMPLE 21

The color image reproduction was conducted by the use of the samereproducing apparatus as that of the Example 19 under the sameconditions of those of the Example 19 except that the charge of -300 Vwas conducted by the pre-writing charger 2 so that an electrostaticimage having a potential of +50 V with respect to the backgroundpotential of -300 V was retained by a first writing operation, that asuperposed voltage composed of a d.c. voltage of -200 V and an a.c.voltage of 2 kHz and 1 kV was applied as a bias for the development tothe developing sleeve 31, and that the pre-writing charger 2 was usedbefore second and later writing operations. The reproduced imageobtained was a color image having an excellent clearness like that ofthe Example 19.

By using the image retaining means having its electrostaticretainability and toner image retainability separated, according to theforegoing Examples of the present invention, there can be attainedexcellent effects that the color tone and so on of the color image canbe easily changed, and that the color image having the excellentclearness and a high tone can be reproduced so that the reproduction canbe stably effected.

Incidentally, the present invention can be applied to the case, in whichthe image retainer has a belt or sheet shape, or to such an imageretainer, e.g., electrofax paper as is placed on a base as can fixwithout any transfer the color image retained thereon by the toners. Inthis case, it is highly necessary to consider the superposed order ofthe color toners, but there arises an advantage that the pre-transferlamp, the transfer means and the cleaning means can be dispensed with.Despite of this fact, the pre-transfer lamp and the charge eliminatingmeans can be omitted in case the toners have predetermined polaritiesand quantities of charges so that they can be transferred. On the otherhand, the transfer may be not only the corona type but also a biasroller type, an adhesion type and a pressure type through anintermediate transfer member. It is quite natural that the fixingoperation should not be limited to a heat roller type.

Although the Examples of the present invention thus far described usedthe electrostatic recording head as the writing means, moreover, othermeans can be similarly used if it can retain the electrostatic chargeimage on the dielectric layer. More specifically, there can be likewiseapplied either a method in which the passage rate of a corona ion flowis controlled by control electrodes so that an electrostatic image maybe retained on the dielectric layer, or a method in which a screenphotosensitive member is used so that the electrostatic charge patternretained thereon may be used for controlling the passage rate of thecorona ion flow to retain the electrostatic image on the dielectriclayer.

Other Examples of the present invention will be described in thefollowing. In the method of consecutively superposing toner images byrepeating the step of retaining a latent image on an image carrier andthe step of developing the retained latent image, as has been describedas the prior art, a development in a suitable density has to beconducted without disturbing the toner image which was retained in theimage carrier at the previous step. Here, the term "superposition" meansnot only that the toner images are formed plural times in an identicalposition of the developing regions of the image carrier but also thatthe toner images are retained in plural times in another portion of theimage region. The result of our investigations has revealed that anexcellent image cannot be obtained even if the values such as the gap d(mm) (which may be simply called the "gap" in the following) between theimage carrier and a developer carrier and the voltage V_(AC) andfrequency f of the a.c. component of the developing bias are satisfiedso as to satisfy the above-specified conditions, and that thoseparameters have close relationships to each other. Therefore,experiments have been conducted by the developing means 16, as shown inFIG. 31, with the parameters such as the voltage and frequency of thea.c. component of the developing bias being changed, so that theresults, as shown in FIGS. 32 and 33, have been obtained. Incidentally,the toner image is previously formed on the photosensitive drum 1 actingas the image carrier drum. The developing means 16 carries a developer Din the direction of arrow B on the circumference of the sleeve 31 tosupply the developer D to a developing region E as a result that thesleeve 31 acting as the developer carrier and the magnetic roll 32 arerotated. Incidentally, the developer D is a two-component developercomposed of a magnetic carrier and a non-magnetic toner. Said carrier iscomposed of ball-shaped particles which have an average particlediameter of 30 μm (which is a weight-averaged value measured by means ofthe Omnicon Alpha (manufactured by Bausch & Lomb Inc.) or the.[.Caulter.]. .Iadd.Coulter .Iaddend.Counter (manufactured by.[.Caulter.]. .Iadd.Coulter .Iaddend.Inc.), a magnetization of 50 emu/gand a resistivity of 10¹⁴ Ω or more and which are coated with a resin.Incidentally, the resistivity is a value which is obtained by readingout a current value when a load of 1 kg/cm² is applied to the tappedparticles so that the carrier particles have a thickness of 1 mm afterthe particles have been tapped in a container having an effectivesectional area of 0.50 cm² and when a voltage for establishing anelectric field of 1,000 V/cm is applied between the load and the bottomelectrodes. Said toner is prepared by adding a small quantity of anelectrification controlling agent to 90 wt. % of a thermoplastic resinand 10 wt. % of a pigment (e.g., Carbon Black) and by blending andpulverizing the mixture so that the particles may have an averageparticle diameter of 10 μm. The developer D is carried in the directionof arrow B by rotating the magnetic roll 32 in the direction of arrow Aand the sleeve 31 in the direction of the arrow B. The developer D hasits thickness regulated in its carrying course by means of the headregulating blade 33. A developer reservoir 47 is equipped therein withan agitating screw 35 so that the developer D may be sufficientlyagitated. When the developer D in the developer reservoir 47 isconsumed, its supply is made from the toner hopper 37 by rotating thetoner supply roller 38.

Between the sleeve 31 and the photosensitive drum 1, moreover, there isconnected a d.c. power supply 45 for applying the developing bias. Inorder that the developer D may be vibrated in the developing region E tobe sufficiently supplied to the photosensitive drum 1, an a.c. powersupply 46 is connected in series with the d.c. power supply 45.Reference numeral 40 is the protecting resistor.

FIG. 32 shows the relationship between the amplitude of the a.c.component, when the gap d between the photosensitive drum 1 and thesleeve 31 is set at 1.0 mm; the thickness of the developer at 0.5 mm;when the charged potential of the photosensitive drum at 600 V; and thedeveloping bias has its d.c. component at 500 V and its a.c. componentat a frequency of 1 kHz, and the image density of a toner image which isformed by the reverse phenomenon on the exposed portion (at a potentialof 0 V) of the photosensitive drum 1. The amplitude E_(AC) of theintensity of the a.c. electric field takes a value which is made bydividing the a.c. voltage of the developing bias by the gap d. Curves A,B and C appearing in FIG. 32 are the results obtained in case the tonersused are controlled to have average charges of 30 μc/g, 20 μc/g and 15μc/g, respectively. It is observed from the three curves A, B and C thatthe effect of the a.c. component appears for the amplitude of the a.c.component of the electric field of 200 V/mm or larger, and that thetoner image retained in advance on the photosensitive drum is partiallybroken for the amplitudes of 2,500 V/mm or larger.

FIG. 33 shows the changes in the image density when the frequency of thea.c. component of the developing bias is set at 2.5 kHz and when thea.c. field intensity E_(AC) is changed under the same conditions ofthose of the experiments of FIG. 32.

According to these experiments, the image density is high when theamplitude E_(AC) of the a.c. field intensity exceeds 500 V/mm, and thetoner image retained in advance on the photosensitive drum 1 ispartially broken when that amplitude exceeds 4 KV/mm, although notshown.

Incidentally, as being seen from the results of FIGS. 32 and 33, theimage density highly changes across a certain amplitude, which has avalue obtainable hardly in dependence upon the average charges of thetoners, as seen from the curves A, B and C. The reason therefor can bethought, as follows. In the two-component developer, specifically, it ispredicted that the toners are charged by the friction with the carrieror by the mutual frictions with one another, and that the charges of thetoners distribute over a wide range, and it is thought that tonershaving a large quantity of charges are preferably developed. Even if theaverage charges are controlled by the electrification controlling agent,the ratio occupied by those toners having the large quantity of chargesdoes not change so much. As a result, it is thought that the changes inthe developing characteristics are found more or less but not highlyobserved.

Now, experiments similar to those of FIGS. 32 and 33 were conductedunder changing conditions to pigeon-hole the relationship between theamplitude E_(AC) and frequency f of the a.c. field intentisy so that theresults shown in FIG. 34 could be obtained.

In FIG. 34: indicated at A is a region where a developing unevenness isliable to occur; indicated at B is a region where the effect of the a.c.component does not appear; indicated at C is a region where the tonersare liable to return, i.e., where the color mixing is liable to occur;and indicated at D and E are regions where the effect of the a.c.component appears so that no color mixing occurs.

These results indicate that a proper region for the amplitude andfrequency of the intensity of the a.c. electric field exists so that anext (or subsequent) toner image may be developed in a proper densitywithout breaking the toner image which was retained at the previous stepon the photosensitive drum 1. This is thought to be explained by thefollowing reasons.

In the region where the image density has a tendency to increase for theamplitude E_(AC) of the a.c. field intensity, e.g., for the densitycurve of FIG. 32, i.e., where the amplitude of E_(AC) of the a.c. fieldintensity ranges from 0.2 to 1.2 KV/mm, the a.c. component of thedeveloping bias acts to make it liable to jump a threshold value atwhich the toners fly from the sleeve. As a result, even the toner havinga small quantity of charges is trapped by the photosensitive drum 1 sothat it can be used for .[.rhe.]. .Iadd.the .Iaddend.development. As aresult, the image density is increased to the higher level as theamplitude of the a.c. field intensity becomes the larger.

For the region where the image density is saturated for the amplitudeE_(AC), i.e., where the amplitude E_(AC) exceeds 1.2 KV/mm in the curveA of FIG. 32, this phenomena can be explained as follows. In thisregion, more specifically, the toners are the more intensely vibrated asthe amplitude of the a.c. field intensity becomes the larger, and thecluster formed as a result of the aggregation of the toners becomesliable to be broken so that only the toners having high charges areselectively applied to the photosensitive drum 1 whereas the tonerparticles having low charges become reluctant to be developed. Moreover,the toners having low charges are liable to be returned to the sleeve 31by the a.c. bias because they have a weak image forming force even ifthey are once trapped by the photosensitive drum 1. Since the charges onthe surface of the photosensitive drum 1 leak because the amplitude ofthe field intensity of the a.c. component, still moreover, thephenomenon that the toners become reluctant to be developed becomeliable to occur. As a matter of fact that, it is thought that thosecauses are overlapped to make the image density constant for theincrease in the a.c. component.

If the a.c. field intensity is raised to have an amplitude exceeding 2.5KV/mm under the condition of obtaining the curve A of FIG. 32, forexample, it is found that the toner image retained in advance on thephotosensitive drum 1 is broken, and that the degree of this breakage isthe higher for the higher a.c. component. This is thought to be causedby the fact that such a force is applied on the toners trapped by thephotosensitive drum 1 as to return to the sleeve 31 by the a.c.component.

In case the development is conducted by consecutively superposing tonerimages on the photosensitive drum 1, it is a fatal problem that thetoner image or images having already been retained are broken at asubsequent developing step.

As is seen by comparing the results of FIGS. 32 and 33, on the otherhand, the experiments conducted by changing the frequency of the a.c.component have revealed that the image density becomes the lower for thehigher frequency. This is caused by the fact that the toner particleshave their vibrating range narrowed, because they cannot follow thechanges in the electric field, so that they become reluctant to betrapped by the photosensitive drum 1.

On the basis of the experimental results thus far described, theInventors have attained a conclusion that a later development can beconducted in a proper density without disturbing the toner image alreadyhaving been retained on the photosensitive drum 1, if each developmentis conducted under the conditions satisfying the following relationshipwhen the amplitude of the a.c. component of the developing bias isdesignated at V_(AC) (V); the frequency of the same at f (Hz); and thegap between the photosensitive sleeve 1 and the sleeve at d (mm).

    0.2≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦1.0.

In order to ensure a sufficient image density and not to disturb thetoner image having been retained by the previous step, it is preferablethat the relationships of the above-specified conditions be satisfied:

    0.5≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦1.0.

If especially the following relationships of the above are satisfied, itis possible to obtain a multi-color image having a better clearness butno color vagueness and to prevent the toner of another color from beingmixed into the developing apparatus even with a number of operations:

    0.5≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦0.8.

Moreover, it is further preferable to set the frequency of the a.c.component at 200 Hz or higher so as to prevent the developing unevennessdue to the a.c. component and to set the frequency of the a.c. componentat 500 Hz or higher so as to eliminate the influences from the beats,which are caused by the a.c. component and by the rotations of themagnetic roll in case the rotating magnetic roll is used as the meansfor supplying the developer to the photosensitive drum 1.

According to the construction of the present invention thus fardescribed, in order to consecutively develop the subsequent toner imagesin predetermined densities on the photosensitive drum without breakingthe toner images retained on the photosensitive drum 1, it is furtherpreferable to use either solely or in suitable combination the followingmethods in accordance with the repetitions of the developments:

(1) toners having consecutively higher charges are used;

(2) the amplitude of the field intensities of the a.c., component of thedeveloping bias are made consecutively smaller; and

(3) the frequencies of the a.c. component of the developing bias aremade consecutively higher.

In other words, the toner particles having the higher charges are themore susceptible to the influences of the electric field. As a result,the toner particles having high charges may return to the sleeve at thestep of the subsequent development if they are trapped by thephotosensitive drum 1 at an early development. Therefore, the method (1)is intended to prevent the toners having low charges from returning tothe sleeve at a later development by using those toner particles at theearly development. The method (2) is intended to prevent the tonerparticles, which have already been trapped by the photosensitive drum 1,from returning by making the field intensities consecutively the smallerin accordance with the repetitions of the development (i.e., at thelater steps of developments). As the specific method of consecutivelyweakening the electric field intensity, there is either a method ofconsecutively dropping the voltage of the a.c. component or a method ofmaking the larger the gap d between the photosensitive drum 1 and thesleeve 31 at the later steps of developments. On the other hand, themethod (3) is intended to prevent the toner particles, which havealready been trapped by the photosensitive drum 1 from returning byraising the frequency of the a.c. component consecutively to a higherlevel as the developments are repeated. Some effect can be obtained ifthose methods (1), (2) and (3) are solely used, but a better effect canbe attained, if they are used in combination, for example byconsecutively increasing the toner charges in accordance with therepetitions of the developments with the a.c. bias being consecutivelydropped. In case those three methods are adopted, moreover, proper imagedensity and color balance can be held by adjusting the d.c. biases,respectively.

Other specific Examples practised by the use of the construction thusfar described will be explained in the following with reference to FIGS.35 and 37.

EXAMPLE 22

FIG. 35 is a schematic view showing an essential portion of a colorimage reproducing apparatus. The photosensitive drum 1 having beenuniformly charged by means of the scorotron charger was exposed to theray, which had been guided from the He-Ne laser light source (althoughnot shown) through a rotary polygonal mirror 51 and a focusing lens 52,to retain an electrostatic latent image. This electrostatic latent imagewas developed by the first developing means 5 so that a first tonerimage was retained on the photosensitive drum 1. And, this first tonerimage was charged again by the scorotron charger 2 and exposed withoutbeing transferred to the recording paper so that a second toner imagewas then retained by the second developing means 6. This is repeateduntil a fourth toner image is retained. In other words, the steps of thecharging operation (the second and later ones of which are not alwaysrequired)→the exposure→the development were repeated four times in theform containing no transfer step. After the toner images had been whollyretained on the photosensitive drum 1, the pre-transfer exposing lamp 10irradiated the region, in which the toner image had been retained on thephotosensitive drum 1, to transfer the toner image to the recordingpaper (the path of which is indicated by a broken line), which was fedfrom the paper feeder (although not shown) by the transfer means 11. Therecording paper was heated and fixed by the fixing means 12, which wascomposed of at lest one heated roller, until it was discharged to theoutside of the machine.

On the other hand, the photosensitive drum 1 having ended itstransferring operation had its charges eliminated by the chargeeliminating means 13, which had not been used during the toner imageretention, and was then cleared of the spare toners, which had been lefton the surface thereof, by the cleaning means 14 which had been leftinoperative during the toner image retention.

The color image reproducing apparatus thus far described were caused torepeat the above operations each time its operation button wasdepressed. Incidentally, in the present Example, the photosensitivematerial used was selenium, and the photosensitive drum 1 had a diameterof 120 mm, a circumferential speed of 120 mm/sec and a charged potentialof 600 V. To the developing means 5 and 6 used, there was applied ateach developing time a developing bias which was composed of a d.c.component of 500 V and an a.c. component having an amplitude of 1 KV anda frequency of 1 kHz. The gap d between the photosensitive drum 1 andthe sleeve of each of the developing means was set at 0.8 mm. Moreover,the developer used was a two-component developer which is composed of amagnetic carrier and a non-magnetic toner. As this carrier, there wasused a ball-shaped one which had an average particle size of 30 μm, amagnetization of 50 emu/g and a resistivity of 10¹⁴ Ω or more and whichwas coated with a resin. The toner was prepared by adding a smallquantity of an electrification controlling agent to 90 wt. % of athermoplastic resin and 10 wt. 5 of a pigment. In the developing means5, 6, 7 and 8, respectively, there were used the yellow, Magenta, cyanand black pigments, all of which had an average quantity of charges of20 μc/g and an average particle diameter of 10 μm. The developer usedwas a mixture which was composed of 80 wt. % and 20 wt. % of theabove-specified carrier and toner, respectively. Moreover, at eachdeveloping time the sleeve 31 and the magnetic roll 32 were rotated ineach of the developing means in directions opposite to each other andhad their heads regulated by the magnetic blade so that the developerlayer had a thickness of 0.4 mm.

With the construction thus far described, as has been described above,the toner images were consecutively superposed to form a multi-colorimage. As a result, a visible image having a sufficient density wasobtained with neither breaking the toner images, which had already beenretained on the photosensitive drum 1 at the subsequent development, norany toner of another color being mixed into each of the developingmeans.

The resultant superposed toner images were transferred to and fixed tothe recording paper so that a clearly reproduced image could also beattained. Even after the toner images had been reproduced on a number ofsheets of the transfer paper, moreover, none of other colors were notmixed into each of the developing means. Incidentally, a small quantityof magnetic material was contained in the toner of each developing meansso that the fog of the image could be further prevented by the magneticforce.

EXAMPLE 23

This example was practised by the color image reproducing apparatusshown in FIG. 35, too. The difference from the Example 22 was that boththe gap d between the photosensitive drum 1 and the sleeve and the d.c.component of the developing bias to be applied at the developing timewere different among the developing means. The gaps and the d.c.components were set at 0.5 mm and 450 V, at 0.7 mm and 500 V, at 0.8 mmand 500 V, and at 1.0 mm and 550 V in the developing means 5, 6, 7 and8, respectively. The average quantities of the charges of the toners andthe amplitude and frequency of the a.c. biases were common among thedeveloping means like the Example 22 and were set at 20 μc/g, 1 KV and 1kHz, respectively.

In the present Example, the return of the toners on the photosensitivedrum 1 was prevented by constructing the photosensitive drum 1 and thesleeves of the respective developing means such that the gaps dinbetween were widened the more of the developing sequence, and thebalance of the densities of the respective color toner images was heldby raising the d.c. biases in the developing order.

According to this Example, a clearer image was obtained, and anothercolor was not mixed into each of the developing means even after thereproduction of the multiple sheets.

EXAMPLE 24

This Example was practised by the color image reproducing apparatusshown in FIG. 35, too. The difference from the Example 22 was that thea.c. component and d.c. component of the developing bias to be appliedat the developing time were different among the developing means. Theamplitudes of the a.c. components and the d.c. components were set at1.5 KV and 450 V, at 1.2 KV and 500 V, at 1.0 KV and 520 V, and at 0.8KV and 550 V in the developing means 5, 6, 7 and 8, respectively. Theaverage quantities of the toners, the frequencies of the a.c. biases,and the gaps between the photosensitive drum 1 and the sleeve werecommon among the developing means like the Example 22 and were set at 20μc/g, 1 kHz and 0.8 mm, respectively.

In the present Example, the return of the toners on the photosensitivedrum 1 was prevented by setting the a.c. components at lower levels inthe developing order, and the balance of the densities of the respectivecolor toner images was held by consecutively raising the d.c. biases.

According to the present Example, a clear multicolored image could beobtained without any mixing of another color into each developing meanseven after the reproducing operations of the multiple sheets.

EXAMPLE 25

This Example was also practised by the color image reproducing apparatusshown in FIG. 35.

The developing conditions were such that the amplitudes of the a.c.components of the developing bias applied at the developing time wereall 1 KV for the respective developing means, and such that thefrequencies and the d.c. components of the same were set at 800 Hz and450 V, at 1 kHz and 500 V, at 1.5 kHz and 550 V, and at 2 kHz and 600 Vin the developing means 5, 6, 7 and 8, respectively.

In each developing means, moreover, at the developing time only thesleeve was rotated to supply the developer whereas the internal magnetswere fixed. The head height regulations were conducted by the magneticblade to provide a gap of 0.5 mm so that the developer had a thicknessof 0.2 mm.

The average quantities of the charges of the toners and the gaps betweenthe photosensitive drum 1 and the sleeve were common among therespective developing means and were set at 20 μc/g and 0.8 mm, and theremaining developing conditions and developers were the same as those ofthe Example 22.

In the present Example, the return of the toners on the photosensitivedrum 1 was prevented by increasing the frequencies of the a.c.components in the developing sequence, and the balance of the densitiesof the respective color toner images was held by consecutively raisingthe d.c. biases.

A clear multi-colored image could also be obtained by the presentExample, and another color was not mixed into each developing means evenafter the reproductions of multiple sheets.

FIG. 36 is a flow chart showing the changes in the potential on thephotosensitive drum 1 when the developments are conducted by the colorimage reproducing apparatus of FIG. 35. Reference letters PH and DAindicate the exposed portion and the unexposed portion, respectively.

The photosensitive drum 1 holds a predetermined potential when it ischarged by the scorotron charger 2, and the portion having beenoptically irradiated has its potential dropped when the image exposureis conducted. Next, by applying a bias, which has its d.c. componentsubstantially equal to the potential of that of the unexposed portion,to the developing means, the toner charged positively in the developingmeans is trapped by the exposed portion having a lower potential so thata development is conducted to retain a first visible image. Thepotential at that particular portion rises a little (as indicated at DUPin the drawing) as a result it traps the positive toners. Next, thepotential on the photosensitive drum 1 is so uniformly charged again bythe charger 2 that it is raised to a predetermined potential (asindicated at CUP in the same drawing). Next, if a second image exposureis conducted and if a development is similarly conducted, the toners areapplied to the exposed portion to retain a second visible image. Byrepeating these steps four times, four color visible images are retainedin a superposed manner on the photosensitive drum 1.

In the methods thus far described, the second and later chargingoperations can be omitted. In case these charging operations are notomitted, on the other hand, a charge eliminating step may be insertedbefore each of the charging operations.

All of the three Examples described hereinbefore conduct the reversaldeveloping methods but can be practised by the normal developing method,i.e., the method in which the toners are applied to the unexposedportion to retain toner images. In case the superposed developments areconducted by the normal method, however, it is necessary to introducethe charging step at each time.

EXAMPLE 26

Next, the description to be made in the following is directed to thecase in which the developments were conducted by means of the colorimage reproducing apparatus shown in FIG. 37.

The photosensitive drum 1 was made of a CdS photosensitive member whichhad its surface covered with an insulating layer and had a diameter of120 mm, a circumferential speed of 120 mm/sec, an insulating layerthickness of 20 μm and a photosensitive layer thickness of 30 μm.

First of all, the photosensitive drum 1 had its surface charged to+1,000 V by means of the primary charger 2 while being exposed all overits surface by the action of a lamp L mounted in that charger 2. Thisexposure was conducted so as to facilitate injection of charges into thephotosensitive layer of the photosensitive drum 1. Next, the surface ofthe photosensitive drum 1 was charged to -100 V to reduce the positivecharges on the surface of its insulating lay by means of the secondarycharger 3' having an a.c. component. The photosensitive drum 1 thuscharged to -100 V was subjected to an image exposure with a ray whichwas reflected from the rotary polygonal mirror 51. The portion thusexposed took a plus potential and was developed by the first developingmeans 5 so that a first visible image was retained. Next, thephotosensitive drum 1 was uniformly charged again to -100 V by thesecondary charger 3' and was then subjected to an image exposure so thata second visible image was retained by the second developing means 6.These operations were repeated four times to retain all the visibleimages on the photosensitive drum 1. After that, the pre-transferexposing lamp 10 irradiated the region, in which the visible images ofthe photosensitive drum had been retained, and these visible images weretransferred by the transfer means 11 to the recording paper (the path ofwhich is shown by the broken line), which was fed from the paper feeder(although not shown). The recording paper was heated and fixed by thefixing means 12, which was composed at least one heated roller, until itwas discharged to the outside of the machine.

On the other hand, the photosensitive drum 1 having its transferringoperation completed had its charges eliminated by the charge eliminatingmeans 10 which had not been used during the toner image retention. Afterthat, the photosensitive drum 1 was cleared of the spare toners, whichwere left on its surface, by the action of the cleaning means 14 whichhad been left inoperative during the toner image retention.

The color image reproducing apparatus thus far described repeated theforegoing operations each time its operating button was depressed. Thedeveloping conditions of each developing step were such that thedeveloping bias to be applied at the developing time had its a.c.component set at 1.5 KV and having a frequency of 2 kHz and its d.c.component set at 0 V, and such the gap d between the photosensitive drum1 and the sleeve of each developing means was 0.5 mm. In each developingmeans, at the developing time the sleeve and the magnetic roll wererotated in the same common direction to carry the developer, and thisdeveloper had its layer thickness regulated to 0.3 mm by the action ofthe magnetic blade.

Each of the developers had the same composition as that of the Example22 except that its charge was controlled to -20 μc/g.

With the construction thus far described, the multi-color images wereretained to form a visible image having a sufficient density withneither breakage of the tone images, which had already been retained onthe photosensitive drum 1, nor any mixing of the toner of another colorinto each developing means.

EXAMPLE 27

This example was likewise practised by the color image reproducingapparatus shown in FIG. 37. The difference from the Example 26 islocated in that the average quantities of the developers used and thed.c. component of the developing bias applied at the developing timewere different among the developing means and were set at -10 μc/g and 0V, at -15 μc/g and 0 V, at -20 μc/g and 20 V, and at -40 μc/g and 50 Vin the developing means 5, 6, 7 and 8 respectively. On the contrary, theamplitudes and frequencies of the a.c. bias and the gaps between thephotosensitive drum 1 and the sleeve were common among the respectivedeveloping means like the Example 26 and were set at 1.5 KV, 2 kHz and0.5 mm, respectively.

In the present Example, the return of the toners on the photosensitivedrum 1 was prevented by controlling the electrifications such that theaverage quantities of the charges of the developers had their absolutevalues increased in the developing sequence, and the balance of thedensities among the respective color toner images was held byconsecutively increasing the values of the d.c. biases.

According to the present Example, too, a clear multi-color image wasobtained, and another color was not mixed into each developing means.

EXAMPLE 28

This Example was likewise practised by the color image reproducingapparatus shown in FIG. 37. The difference from the Example 26 was foundin that the average quantities of the developers used and the amplitudesof the a.c. components of the developing biases applied at thedeveloping time were different among the developing means and were setat -10 μc/g and 1.6 KV, at -15 μc/g and 1.4 KV, at -20 μc/g and 1.2 KV,and at -40 μc/g and 1.0 KV in the developing means 5, 6, 7 and 8,respectively. The frequencies of the a.c. biases, the potentials of thed.c. biases, and the gaps d between the photosensitive drum 1 and thesleeve were shared among the respective developing means and were set at2 kHz, 0 V and 0.5 mm, respectively.

In the present Example, the return of the toners on the photosensitivedrum 1 was prevented, and at the same time the balance among thedensities of the respective color toner images was held partly bycontrolling the electrifications such that the average quantities of thecharges of the developers had their absolute values increased and partlyby consecutively setting the a.c. biases.

According to the present Example, a clearer multi-color image wasobtained, and no color was mixed into each developing means even afterreproductions of multiple sheets.

FIG. 38 shows the changes in the potentials on the photosensitive drumwhen the developments are conducted by the color image reproducingapparatus of FIG. 37.

After has been charged positive by the primary charger 2, thephotosensitive drum 1 is charged negative so that its surface potentialis dropped substantially to 0 V. Next by conducting the image exposure,the portion optically irradiated has its potential raised to trap thetoners, which have been charged negative in the developing means, sothat the portion having trapped the toners has its potential dropped (asindicated at DDW in the drawing). Next, a uniform charging operation isso conducted by the secondary charger that the surface potential isdropped substantially to 0 V, and the image exposure and the developmentare repeated. After the visible images of all the colors have beenformed on the photosensitive drum 1, the resultant toner images aretransferred to the recording paper, and the photosensitive drum 1 hasits charged eliminated and is then cleaned until the step advances to asubsequent image reproduction.

In the methods described hereinabove, the second and later secondarycharging operations can be omitted. On the other hand, the primary andsecondary charging operations may be conducted each time, and in thiscase the charge eliminating step may be introduced prior to the chargingstep.

In the respective Examples thus far described, the corona transfer isused as the toner image transfer, but another type may be used. If theadhesion transfer disclosed in Japanese Patent Publication Nos.41679/71, 22763/73 or the like, for example, is used, the transfer canbe conducted without considering the polarities of the toners. Moreover,it is possible to adopt the method of effecting direct fixture to thephotosensitive member as in the electrofax method.

The two-component developer used in the present invention may especiallypreferably be composed of a magnetic carrier as its carrier and anon-magnetic toner as its toner.

The compositions of the toners are generally, as follows:

(1) Thermoplastic Resin: 80 to 90 wt. % of binder

Examples: polystyrene, styrene-acryl polymer, polyester, polyvinylbutyral, epoxy resin, polyamide resin, polyethrene, and ethylene-vinylacetate copolymer, which are frequently used in a mixed form;

(2) Pigment: 0 to 15 wt. % of coloring agent

Examples:

Black: Carbon Black;

Blue: copper phthalocyanine, derivative dye of sulfonamide;

Yellow: benzine derivative; and

Magenta: polytungstophosphate, Rhodamine Lake, Carmine 6B;

(3) Electrification Controlling Agent: 0 to 5 wt. %

Examples:

Plus: Nigrosine (i.e., electron donor); and

Minus: organic complex (i.e., electron acceptor);

(4) Fluidizer:

Examples: coloidal silica or hydrophobic silica as representative,silocone varnish, metallic soap, nonionic active agent;

(5) Cleaning Agent: intended to prevent the filming of the toners ofphotosensitive member

Examples:

fatty acid metal salt, oxidized silicate having a surface radical,surface active agent containing fluorine; and

(6) Filler: intended to improve the surface gloss of images and toreduce the cost for raw materials

Examples:

Calcium carbonate, clay, talc, pigment.

In addition to the above-enumerated materials, a magnetic material maybe contained so as to prevent a fog and a toner dispersion.

As the magnetic powders, there are proposed such powders of tri-irontetraoxide, γ-ferric oxide, chromium dioxide, nickel ferrite or ironalloy as have a diameter of 0.1 to 1 μm. At present, however, thetri-iron tetraoxide is frequently used and is contained in 5 to 7 wt. %with respect to the toners. The resistances of the toners are variablein dependence upon the kinds and quantities of the magnetic powders. Inorder to provide a sufficient resistance, however, it is preferred tocontain 55 wt. % or less of the magnetic material. Moreover, thequantity of the magnetic material is desired to be contained in 30 wt. %or less so that it may hold a clear color as the color toner.

In addition, as the resin suitable for the pressure fixing toner, anadhesive resin such as wax, polyorefines, ethylene-vinyl acetatecopolymer, polyurethane or rubber is selected so that it may beplastically deformed and adhered to paper by a force of about 20 kg/cm.A capsule toner may also be used.

The toners can be made of the above-enumerated materials and prepared bythe method known in the prior art.

In order to obtain a more preferable image in the construction of thepresent invention, the particle diameters of those toners are desired tobe no more than 50 microns in their ordinary average values in relationto the resolution. In the present invention, the toner diameters ofabout 1 to 30 microns may preferably be used in relation to theresolution, the toner scattering and the carriage, although they are notrestricted on principle.

In order to reproduce fine points and lines and to enhance thegradation, moreover, the magnetic carrier particles may preferably beparticles composed of magnetic particles and a resin, for example, aresin-dispersed system of magnetic powders and a resin or resin-coatedmagnetic particles and may more preferably be rounded to have an averageparticle diameter of 50 μm or smaller, especially preferably, a particlediameter no more than 30 μm and no less than 5 μm.

Moreover, in order to prevent the problems that the carrier particlesfor providing an obstruction against the satisfactory image reproductionare made liable to receive the charges by the bias voltage so that theybecome liable to be trapped by the surface of the image carrier and thatthe bias voltage is not applied to a sifficient level, the carrier mayhave such an insulating property of a resistivity no less than 10⁸ Ω,preferably, 10¹³ Ω, more preferably, 10¹⁴ Ω. Moreover, the carrierparticles may have this resistivity and the above-mentioned diameter.

The carrier particles described above can be prepared either by coatingthe surface of the magnetic materials described as to the toners withthe thermoplastic resin or by making the particles of a resin havingfine magnetic particles dispersed and contained therein and by selectingthe resultant particles by the well-known average diameter selectingmeans. Moreover, in order to improve the agitating characteristics ofthe toners and the carriers and the carrying characteristics of thedevelopers and to improve the electrification controllingcharacteristics of the toners thereby to make the toner particlesreluctant to aggregate or the toner particles and the carrier particlesto aggregate, it is desirable to round the carriers. Of these roundedmagnetic carrier particles, the resin-coated ones are prepared byselecting magnetic particles as round as possible and by coating theparticles selected with a resin, and the carriers having fine magneticpowders dispersed therein are prepared either by rounding fine particlesof a magnetic material, if possible, by hot wind or water after makingthe dispersed resin particles or by directly forming the roundeddispersed resin particles by the spray dry method.

Incidentally, the present invention can be further modified on the basisof the technical concept thereof. In the Examples, the description hasbeen made as to the case in which the two-component developer composedof the toner and the carrier was used as the developer having aplurality of components. However, the developer may additionally containa third component.

In the Examples, the description is limited to the development of thecolor image. However, the present invention can be applied to the casein which toners of the same color are developed in plural times. In thiscase, a toner having an excellent gradation can be retained on thephotosensitive drum.

Still moreover, the present invention can be applied not only to thereproducing apparatus by electrophotography but also to the non-impactprinter making use of the electrostatic reproducing method or themagnetic reproducing method.

According to the Examples of the present invention, an image at asubsequent step can be retained on an image carrier without disturbingan image retained at a previous step even if the step of retaining alatent image on the image carrier and the step of developing the latentimage with a developer having a plurality of component are repeated aplurality of times.

In other words, a clear image can be retained on the image carrier ifthe amplitude V_(AC) and the frequency f of the a.c. component and thegap d between the developer carrier and the image carrier are so set asto satisfy the following relathinships:

    0.2≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦1.0.

In the other Examples, the developer D used was a one-component magneticdeveloper which was prepared by blending and pulverizing 70 wt. % of athermoplastic resin, 10 wt. % of a pigment (e.g., Carbon Black), 20 wt.% of a magnetic material and an electrification controlling agent tohave an average particle diameter of 10 μm. The quantity of the chargesis controlled by the electrification controlling agent.

In case the development is conducted with a one-component developerusing only the magnetic or non-magnetic toner, there can be useddeveloping means which is disclosed in U.S. Pat. Nos. 3,866,574 and3,893,418. On the other hand, developing means having two or moremagnetic rollers may be used. The electric bias containing vibratorycomponents and applied upon the development has to be set under such acondition that the toner image retained already on the image retainermay neither be disturbed nor have a color mixing. Under the biascondition used in the non-contact jumping development, e.g., thecondition as is disclosed in Japanese Patent Laid-Open Nos. 18656 to18659/80 and 106253/81, the toner images having already been retainedmay be damaged by the vibrations of the toners, which are caused by theintense a.c. electric field. In case the developments according to thepresent invention are repeated to superpose the toner images, theintensity of the a.c. component of the bias has to be set within such aproper range without deteriorating the retained toner images that asubsequent toner image can be completely retained.

FIG. 39 shows the relationship between the amplitude of the a.c.component, when the gap d between the photosensitive drum 1 and thesleeve 31 is set at 0.7 mm; the thickness of the developer at 0.3 mm;the developing bias to be applied to the sleeve 31 has its d.c.component at 500 V and its a.c. component at a frequency of 1 kHz; andthe charged potential of the photosensitive drum at 600 V, and the imagedensity of a toner image which is formed by the reverse phenomenon onthe exposed portion (at a potential of 0 V) of the photosensitivedrum 1. The amplitude E_(AC) of the intensity of the a.c. electric fieldtakes a value which is made by dividing the a.c. voltage of thedeveloping bias by the gap d. Curves A, B and C appearing in FIG. 39 arethe results obtained in case the magnetic toners used are controlled tohave average charges of 5 μc/g, 3 μc/g and 2 μc/g, respectively. It isobserved from the three curves A, B and C that the effect of the a.c.component appears for the amplitude of the a.c. component of theelectric field of 200 V/m or higher and 1.5 KV/mm or lower, and that thetoner image retained in advance on the photosensitive drum is partiallybroken for the amplitude of 2,500 V/mm or larger.

FIG. 40 depicts the changes in the image density when the frequency ofthe a.c. component of the developing bias is set at 2.5 kHz and when thea.c. field intensity is changed under the same conditions of those ofthe experiments of FIG. 32.

According to these experiments, the image density is high when theamplitude E_(AC) of the a.c. field intensity is 500 V/mm or higher and3.8 KV/mm or lower (although not shown in FIG. 39), and the toner imageretained in advance on the photosensitive drum 1 is partially brokenwhen that amplitude exceeds 3.2 KV/mm (although not shown in FIG. 39).

Incidentally, as being seen from the results of FIGS. 39 and 40, theimage density highly changes across a certain amplitude, which has avalue obtainable hardly in dependence upon the average charges of thetoners, as seen from the curves A, B and C. The reason therefor can bethought, as follows. Specifically, it is predicted that theone-component developer has its charge quantities distributed widelyacross the positive and negative ranges because of the mutual frictionsof the toner particles. As a result, the average quantities of thecharges take a small value, but in fact toners having a large quantityof charges, e.g., 20 μc/g or larger exist at a predetermined ratio andare thought to be mainly developed. Even if the average charge quantityis controlled by the electrification controlling agent, the ratiooccupied by the toners having that large charge quantity is not variedso much, so that it is thought that the change in the developingcharacteristics is not substantially observed.

Now, experiments similar to those of FIGS. 39 and 40 were conductedunder changing conditions to pigeon-hole the relationship between theamplitude E_(AC) and frequency f of the a.c. field intensity so that theresults shown in FIG. 41 could be obtained.

In FIG. 41: indicated at A is a region where a developing unevenness isliable to occur; indicated at B is a region where the effect of the a.c.component does not appear; indicated at C is a region where the tonersare liable to return; and indicated at D and E are regions where theeffect of the a.c. component appears so that no toner return occurs.

These results indicate that a proper region for the amplitude andfrequency of the intensity of the a.c. electric field exists so that anext (or subsequent) toner image may be developed in a proper densitywithout breaking the toner image which was retained previously (at theprevious step) on the photosensitive drum 1. This is thought to beexplained by the following reasons.

In the region where the image density has a tendency to increase for theamplitude E_(AC) of the a.c. field intensity, e.g., for the densitycurve of FIG. 39, i.e., where the amplitude of E_(AC) of the a.c. fieldintensity ranges from 0.2 to 1.0 KV/mm, the a.c. component of thedeveloping bias acts to make it liable to jump a threshold value atwhich the toners fly from the sleeve. As a result, even the toner havinga small quantity of charges is trapped by the photosensitive drum 1 sothat it can be used for the development. As a result, the image densityis increased to the higher level as the amplitude of the a.c. fieldintensity becomes the larger.

On the other hand, the reason, for which the image density is dropped inaccordance with the increase in the amplitude of the a.c. electric field(e.g., the region in which the amplitude E_(AC) of the a.c. fieldintensity is no less than 1 KV for the density curve A of FIG. 29), canbe thought in several ways. The toners are the more intensely vibratedas the amplitude E_(AC) of the a.c. field intensity becomes the larger,and the cluster formed as a result of the aggregation of the tonersbecomes liable to be broken so that only the toners having high chargesare selectively applied to the photosensitive drum 1 whereas the tonerparticles having low charges become reluctant to be developed. Moreover,the toners having low charges are liable to be returned to the sleeve 31by the a.c. bias because they have a weak image forming force even ifthey are once trapped by the photosensitive drum 1. Since the charges onthe surface of the photosensitive drum 1 leak if the amplitude of thefield intensity of the a.c. component is too large, still moreover, thephonomenon that the toners become reluctant to be developed becomeliable to occur. As a matter of fact that, it is thought that thosecauses are overlapped to make the image density constant for theincrease in the a.c. component.

If the amplitude E_(AC) of the a.c. field intensity is enlarged, as hasbeen described hereinbefore, on the other hand, the toner image retainedin advance on the photosensitive drum 1 is broken, and the degree ofthis breakage is the higher for the higher a.c. component. This isthought to be caused by the fact that the toners trapped by thephotosensitive drum 1 are acted by a force for returning it to thesleeve 31 by the a.c. component. In case the development is conducted byconsecutively superposing toner images on the photosensitive drum 1, itis a fatal problem that the toner image or images having already beenretained are broken at a subsequent developing step.

As seen by comparing the results of FIGS. 39 and 40, on the other hand,the experiments conducted by changing the frequency of the a.c.component have revealed that the image density becomes the lower for thehigher frequency. This is caused by the fact that the toner particleshave their vibrating range narrowed, because they cannot follow thechanges in the electric field, so that they become reluctant to betrapped by the photosensitive drum 1.

On the basis of the experimental results thus far described, theInventors have attained a conclusion that a later development can beconducted in a proper density without disturbing the toner image alreadyhaving been retained on the photosensitive drum 1, if each developmentis conducted under the conditions satisfying the following relationshipswhen the amplitude of the a.c. component of the developing bias isdesignated at V_(AC) (V); the frequency of the same at f (Hz); and thegap between the photosensitive drum 1 and the sleeve at d (mm):

    0.2≦V.sub.AC /d·f≦1.6

In order to obtain a sufficient image density but not to disturb thetoner images having been retained until the previous step, the followingcondition, i.e., the region of FIGS. 29 and 30, in which the imagedensity has a tendency to increase for the a.c. electric field, isdesirably satisfied:

    0.4≦V.sub.AC /d·f≦1.2.

Of this region, it is preferable to satisfy the following regioncorresponding to a slightly lower electric field in which the imagedensity takes its maximum:

    0.6≦V.sub.AC /d·f≦1.0.

Moreover, it is further preferable to set the frequency f of the a.c.component at 200 Hz or higher so as to prevent the developing unevennessdue to the a.c. component and to set the frequency of the a.c. componentat 500 Hz or higher so as to eliminate the influences from the beats,which are caused by the a.c. component and by the rotations of themagnetic roll in case the rotating magnetic roll is used as the meansfor supplying the developer to the photosensitive drum 1.

On the other hand, not only the magnetic toner but also a non-magnetictoner can be used. As the developing method using the non-magnetictoner, there is known a method which is disclosed in Japanese PatentLaid-Open No. 30537/75 or 22926/77, for example. In order to easilytransfer the visible image on the photosensitive drum 1 to the recordingpaper, moreover, the specific resistance of the toner is desired to beno less than 10¹³ Ω cm. Incidentally, the resistivity is a value whichcan be obtained by reading out a current value when a load of 1 Kg/cm²is applied to the particles tapped in a container having an effectivearea of 0.5 cm² and when a voltage for establishing an electric field of1,000 V/cm is applied between the load and the bottom electrodes.

Moreover, the materials composing the developer except the magneticmaterial are similar to those of the foregoing Examples.

These materials may be simply blended and pulverized, but the followingadditional devices are made, as the case may be:

1. An insulating material is added to the inside or surface of thetoner.

2. The toner is prepared either by coating in advance the surfaces ofmagnetic powders with a surface active agent, an organic dye or aspecified resin or by activating in advance the same surfaces to formcover films by polymerizations and by mixing the magnetic powders with aresin or the like. This device is intended to facilitate uniformdispersion into the resin and to improve the image quality in a highhumidity.

3. The developing quality is improved to prevent the toner scatter, asthe case may be, by selecting the magnetic characteristics of themagnetic powders such as the shape, the axial ratio or the retainingforce of the same.

4. The fluidity is enhanced to improve the developing property by mixingmagnetic toners which have different particle diameters, quantities ofmagnetic powders contained, magnetic characteristics and electricresistances.

On the other hand, most of the magnetic powders are black so that theycan be used in place of the black pigment.

In addition, as the resin suitable for the pressure-sensitive toner,wax, polyorefines, ethylenevinyl acetate copolymer, polyurethan, rubberand so on are selected such that they are elastically deformed andadhered to the paper by a force of about 20 Kg/cm². Capsulated tonersmay also be used.

The particle diameters of those toners may preferably be no more than 50microns on an average value in relation to the resolution. In thepresent invention, the toner particle diameters are not limited onprinciple but may be ordinally about 1 to 30 microns in relation to theresolution and the scattering and carriage of the toners.

Incidentally, the present invention can be further modified on the basisof the technical concept thereof. In the foregoing Examples, thedescription is restricted to the development of the color image. Thepresent invention can also be applied to the case in which toners of thesame color are developed in plural times. In this case, a toner imagehaving an excellent gradation can be retained on the photosensitivedrum.

Moreover, the present invention can be applied not only to the recordingmethod for electrophotography but also the non-impact printer whichmakes use of the electrostatic reproducing method or the magneticproducing method.

Even both the step of retaining the latent image on the same imagecarrier and the step of developing the latent image with theone-component developer are repeated plural times, according to theExamples of the present invention, an image at a subsequent step can beretained on the image carrier without disturbing the image which hasbeen retained at a previous step.

In other words, a clear image can be retained on the image carrier ifthe amplitude V_(AC) and the frequency f of the a.c. component and thegap d between the developer carrier and the image carrier are so set asto satisfy the following relationship:

    0.2≦V.sub.AC /d·f≦1.6.

FIG. 42 shows a reproducing apparatus according to another embodiment ofthe present invention, in which: reference numeral 61 indicates an imageretainer which is constructed of such a magnetic layer and an insulatinglayer as is prepared by evaporating or sputtering a magnetic material ona metal base or by applying a magnetic material dispersed in a binder tothe metal base and which is placed on a drum rotating in the directionof arrow; numeral 63' indicates a magnetic erasing head; and numeral 63indicates a magnetic recording head. The remaining portions areidentical to the embodiment of FIG. 1.

The recording head 63 composed of one or more rows of recording headsfor retaining a magnetic image on the magnetic layer of the imageretainer 61. Magnetic force of the magnet 32 is arranged so as not todisturb a magnetic image on the retainer 61.

The bias conditions for practising the method of the present inventionare preferred to satisfy the following inequalities:

For the two-component developer used:

    0.2≦V.sub.AC /(d·f); and

    {(V.sub.AC /d)-1500}/f≦1.0; and

For the one-component developer used:

    0.2≦V.sub.AC /d·f≦1.6.

In the above inequalities: V_(AC) indicates the amplitude (V) (althoughnot an effective value) of the a.c. component of the developing bias; findicates the frequency (Hz); and d indicates the gap (mm) between theimage retainer, e.g., the sleeve and the developer carrier.

Moreover, the order of the colored toners for image superpositions hasto be determined to be the most proper for the object because it exertsinfluences upon the tone of the color image.

Although the foregoing description is directed to the reproducingapparatus of FIG. 42, the method of the present invention can also bepractised by the reproducing apparatus shown in FIG. 43. In FIG. 43,parts having the same functions as those of FIG. 42 are indicated by thesame reference numerals as those of FIG. 42.

Shown in FIG. 43 is the reproducing apparatus in which a series ofrecording members are prepared by placing a magnetic layer and acolorless insulating layer on the surface of a conductive base toprovide an image retainer 61'. While this image retainer 61' is beingfed straight, the retentions and developments of the magnetic images arerepeated. Along the passage for the image retainer 61', morespecifically, the pre-writing charger 2, the magnetic erasing head 63',the magnetic recording head 63 and developing means 5 to 8 arejuxtaposed in a repeated manner, and the fixer 12 for fixing the colorimage on the image retainer 61' is disposed at the last position. Thisreproducing apparatus can reproduce a series of color images without anyprovision of the pre-transfer charger, the transfer means, the chargeeliminating means and the cleaning means. In order that the imageretainer 61' may not depend, it is necessary to increase the tension orto provide such a supporting roller midway, although not shown, asproviding the toners trapped by the image retainer 61' from beingoffset.

In the recording apparatus shown in FIG. 42, too, the pre-transfercharger 9, the transfer means 11, the charge eliminating means 13 andthe cleaning means 14 can be omitted if the image retainer 61 isconstructed by winding on the drum an image retainer which is similar tothe image retainer 61' used in the reproducing apparatus of FIG. 43. Inorder to hide the color of the magnetic layer, moreover, it is desiredto provide a conductive layer or an insulating layer having a white ordesirable color.

In order to .[.practise.]. .Iadd.practice .Iaddend.the method of thepresent invention, it is preferable to use the image retainer having ahighly insulating layer as the image retainer. Once the developer istrapped by the image retainer, generally speaking, it is remarkablydifficult to remove, because not only the van der Waals' force but alsothe image forming force acts, to cause troubles such as the fog and thereduction in the transfer ratio. These phenomena can be prevented bysuitably charging the image retainer in the same polarity as that of thecharges of the toners. However, the ordinary magnetic image retainerconstructed of a conductive base and a magnetic layer has such a lowinsulating property that it is difficult to charge. Despite of thisfact, this ordinary image retainer can be charged by forming aninsulating layer on the surface of the magnetic layer. By using theimage retainer having the insulating layer on the surface of themagnetic member, the unnecessary trap of the toners can be prevented toenhance the transfer efficiency, and a charger is placed in front of themagnetic image writing operation to effect the charging operation sothat the fog can be prevented. Moreover, the insulating layer is alsoeffective for protecting the magnetic layer and for preventing the tonerfilming. If this toner filming occurs, there arises no practical problemif it takes place on the insulating layer. Moreover, the magnetic layermay also act as the conductive base if it is conductive. In case thethickness of the insulating layer is excessive, it drops the density andmagnetization of the magnetic image recorded. Therefore, that thicknessis preferred to be no more than 50 μm or, preferably, no more than 10μm.

Incidentally, FIG. 42 shows an embodiment of the reproducing apparatuswhich uses an image retainer having the insulating layer, but thepre-writing charger 2 and the charge eliminating means 13 can be omittedin case an image retainer having no insulating layer is used.

In the embodiment of FIG. 42, the writing operation of the magneticimage is conducted by the reproducing method of parallel magnetizationtype using a ring head. However, a perpendicular magnetization typemethod can be likewise used as the magnetically writing means. In thiscase, the magnet 32 is fixed, and its opposed magnetic poles are madedifferent from the magnetizing direction by the writing operation sothat the toners may be reluctant to jump to the non-image portion butliable to jump to the image portion. In this case, it is needless to saythat the magnetizing direction and the magnetization facilitatingdirection of the magnetic layer should be aligned.

On the other hand, the magnetic erasing head 63' and the magneticrecording head 63 may be disposed in front of, in front of and at theback of, or at the back of the pre-writing charger as shown in FIGS. 42and 43.

The image reproducing process using the reproducing apparatus accordingto the method of the present invention will be described in thefollowing with reference to FIGS. 44 to 46.

In the embodiment of FIG. 44, by the reproducing apparatus of FIG. 42:(1) the surface of the image retainer 61 has its charges eliminated bythe image eliminating means 13 and cleaned by the cleaning means 14; andthe initial state, in which the surface of the image retainer 61' ischarged to a suitable potential e (in which the broken lines indicatethe presence of the charges) in the same polarity as that of the toners,is established by the pre-writing charger 2 so as to prevent the fog.

Next, after the residual magneticism m has been demagnetized by thedemagnetizing head 63' (shown at (3)), that surface is subjected to afirst writing operation by the recording head 63 to retain a magneticimage M₁ (shown at 4)) which is firstly developed by the developingmeans 5 to obtain a first image T₁ (shown at (5)). Moreover, the imageretainer 1 enters its second rotation in the reproducing apparatus ofFIG. 42 so that it is demagnetized by the erasing head 63' (shown at(6)) and is subjected to a second writing operation by the recordinghead 63 (shown at (7)). The magnetic image M₂ thus formed is secondlydeveloped by the developing means 6 to provide a second image T₂ (shownat (8)). Then, third and fourth demagnetizing, writing operations anddevelopments are likewise repeated so that a color image having itscolor toner images superposed is retained on the image retainer 61. Theresultant color image is made liable to be transferred by thepre-transfer charger 9 in the reproducing apparatus of FIG. 42 so thatit is fixed on the recording member P by the fixing means 12 after ithas been transferred to the recording member P by the transfer means 11.In the reproducing apparatus of FIG. 43, on the other hand, that colorimage is fixed directly on the image retainer 61' by the fixing means12. In the reproducing apparatus of FIG. 42, moreover, the surface ofthe image retainer 61 having the color image transferred thereto has itscharges eliminated by the charge eliminating means 13 and is cleared ofthe residual toners by the cleaning means 14 until the one cycle of thecolor image reproduction is ended by further eliminating the charges, ifnecessary. In case the reproducing apparatus of FIG. 43 is used, too,the image retaining process is not changed except for the shape of theimage retainer 61'.

FIG. 45 shows a process which is simplified by omitting the uniformlycharging step from the process of FIG. 44. FIG. 46 shows a process whichis difference from that of FIG. 44 in that the charger 2 is operated toeffect the uniform charging operation before each writing step. However,their basic operations are all common.

Incidentally, reference letters T₁ and T₂ indicate the toners ofdifferent colors, which are trapped by the image retainer 61 or 61'.

By conducting the developments under the non-contact jumping developingconditions, according to the method of the present invention, thedeveloping means other than that conducting the development of each timecan be easily held in an inoperative state, even if the developer layeris not removed from the developing sleeve 31, by disconnecting thedeveloping sleeve 31 from the power supply 39 into a floating state, bygrounding the same to the earth, or by positively applying a d.c. biasvoltage having a polarity opposite to that of the charges of the tonersto the developing sleeve 31. Of these means, it is preferable that thedeveloping means are held inoperative by applying the bias voltagehaving a polarity opposite to that of the toners.

Next, the embodiments of FIGS. 44 to 46, which are practised by thereproducing apparatus of FIG. 42, will be described in more detail inthe following in connection with Examples 29 to 31.

The reproducing apparatus shown in FIG. 42 was used. The image retainer61 was prepared by forming a Co alloy having a thickness of 10 μm on analuminum base by the electron beam heating operation and by forming theinsulating layer having a thickness of 5 μm on the surface of the Coalloy and which had a circumference speed of 180 mm/sec. The surface ofthe image retainer 61 thus prepared was charged to +50 V by thepre-writing charger 2 using the scorotron corona discharger and wasdemagnetized by means of the magnetic erasing head which had its leadingend spaced at a distance of about 30 μm from the surface of the imageretainer 61. Next, a first image writing operation was conducted in adistribution density of 10 spots/mm by means of the recording head 63which had a similar spacing. As a result, a first magnetic image wasretained on the image retainer 61. This magnetic image was firstlydeveloped by the developing means 6 shown in FIG. 3. This developingmeans 6 uses the developer, which was composed of: a carrier having 50wt. % of magnetite dispersed contained in a resin and having an averageparticle diameter of 30 μm, a magnetization of 30 emu/g and aresistivity of 10¹⁴ Ω cm or more; and a positive magnetic toner preparedby adding 25 wt. % of magnetite, 10 wt. % of copper phthalocyanine asthe cyan pigment and an electrification controlling agent to astyreneacryl resin and having an average particle diameter of 10 μm,under the condition that the ratio of the toner to the carrier was 10wt. %. Moreover, there were resorted to the non-contact jumpingdeveloping conditions under which: the developing sleeve 31 had anexternal diameter of 30 mm and a number of revolutions of 100 r.p.m.;the magnet 32 and its N and S magnetic poles having a magnetic fluxdensity of 500 gausses and had a number of revolutions of 1,000 r.p.m.;the developer layer in the developing region had a thickness of 0.7 mm;the gap between the developing sleeve 31 and the image retainer 1 was0.8 mm; and a bias voltage having a d.c. voltage component of -50 V andan a.c. voltage component of 1.5 kHz and 1,000 V was applied to thedeveloping sleeve 31. In the following Examples, the a.c. component hasa sine wave, and its exemplified values are effective ones.

The surface of the image retainer 61 having been subjected to the firstdevelopment was subjected again to an erasure by the same magneticerasing head 63' without operating the pre-transfer charger 9, thecharge eliminating means 13 and 13, the cleaning means 14 and thepre-writing charger 2, and a second writing operation was conducted inthe same spot density but with the spot position being shifted from thatof the first writing operation by means of the recording head 63. Next,a second development was conducted by the developing means 6 which wasunder the same conditions as those of the developing means 5 except thatit used as its toner a toner prepared by adding polytungstophosphate asthe .[.Magenta.]. .Iadd.magenta .Iaddend.pigment in place of the cyanpigment. Likewise, a demagnetization and a third writing operation wereconducted. A third development was then conducted by the developingmeans 7 which was under the same conditions as those of the developingmeans 5 except that it used as its toner a toner prepared by adding abendizine derivative as the yellow pigment. Moreover, demagnetizationand a fourth writing operation were conducted. A fourth development wasconducted by the developing means 8 which was under the same conditionsas those of the developing 5 except that it used a toner prepared byadding .[.Carbon Black.]. .Iadd.carbon black .Iaddend.as the blackpigment. The color image thus retained on the image retainer 61 wastransferred to and fixed on the recording member P, as has beendescribed with reference to FIG. 42. Moreover, the surface of the imageretainer 61 having the color image transferred thereto had its chargedeliminated by the charge eliminating means 13 and was cleared of theresidual toners by the cleaning means 14.

The reproduced image thus obtained had little color toner mixing and wasa remarkably clear color image.

Incidentally, in the present Example, the spot position of a subsequentwriting operation may be overlapped upon that of a previous writingoperation. In the writing and developing operations, moreover, therecording current of the recording head 3, and the voltage value,frequency and time selecting period of the d.c. or a.c. component of thevoltage to be applied to the developing sleeve may be so changed as toadjust the developed densities of the respective colors. If the writingspot positions are superposed, the color mixing becomes liable to occurto invite color vagueness but not to drop the resolution. In this case,moreover, especially the sequence of the colors to be developed isimportant. By adjusting the developing densities of the respectivecolors in the aforementioned manner, on the other hand, it is possibleto attain a color image which has its tones changed.

EXAMPLE 30

The same reproducing apparatus as that of the Example 29 was used. Thecolor image reproduction was conducted under the same conditions asthose of the Example 29 except: that a magnetic image was for abackground potential of 0 V by a first writing operation without any ofthe charging operation of the Example 29 by the pre-writing charger 2before the first writing operation, after the charge elimination fordemagnetization by the charge eliminating means 13; that a superposedvoltage composed of a d.c. voltage of -50 V and an a.c. voltage of 3 kHzand 2,000 V was applied as the bias voltage before development to thedeveloping sleeve 31; and that charge elimination and demagnetizationwere effected by the charge eliminating means 13 before second and laterwriting operations so that a magnetic image was retained for thebackground potential of 0 V even during the second and later writingoperations.

The reproduced image which is excellent in clearness like that of theExample 29 was thus obtained.

EXAMPLE 31

The color image reproduction was conducted by the use of the samereproducing apparatus as that of the Example 29 and under the sameconditions as those of the Example 29 except: that a charging operationis conducted to +300 V before a first writing operation by thepre-writing charger 2 so that a magnetic image was retained for thebackground potential of +300 V by the first writing operation after ithad been demagnetized; that a superposed voltage composed of a d.c.voltage of +300 V and an a.c. voltage of 2 kHz and 1 KV was applied asthe bias before the development to the developing sleeve 31; and thatthe pre-writing charger 2 was used before the demagnetization and thesecond and later writing operations. The reproduced image obtained was acolor image which was excellent in cleaness like that of the Example 29.

According to the Examples of the present invention, there can beattained an excellent effect that the tone or the like of the colorimage can be easily changed thanks to the use of the image retainingmeans having its image retainability and toner image formabilityseparated so that a color image having excellent clearness and a hightone can be reproduced while stabilizing the reproduction.

Incidentally, the present invention can be applied not only to an imageretainer having a belt or sheet shape, but also to an image retainersuch as electrofax paper, which is placed on a base so that the colorimage formed thereon by the toners is fixed without being transferred.In this case, the sequence of superposing the color toner images has tobe taken into consideration, but the transfer means, the cleaning meansand so on can be omitted. It is true, but the the charge eliminatingmeans can be omitted, too, in case the toners are transferred with apredetermined polarity and a charge quantity. On the other hand, thetransfer should not be limited to the corona type but may be exemplifiedby the bias roller type, the adhesion type, the direct pressure type orother means using an intermediate transfer member, and the fixture isnot limited to the heat roller type.

In the foregoing embodiments of the present invention, moreover, themagnetic recording head is used as the writing means, but another meansmay be likewise used if the magnetic image is to be retained on themagnetic layer. More specifically, the present invention can be appliedto the method, in which the magnetic image is retained by heatingimagewise a demagnetized magnetic layer, while passing through a unidormmagnetic field, by the heating means such as a laser and by cooling theheated magnetic layer in a magnetic field.

Although the foregoing description has been directed only to thereproduction of the color image, furthermore, the method of the presentinvention can also be applied to the superposition of an image ofidentical colors. In addition, the electrophotographic image and themagnetic image can be reproduced in combination if anelectrophotogtaphic photosensitive layer is provided on the magneticlayer.

What is claimed is: .[.
 1. An apparatus for reproducing multiplex imagescomprising an image retainer having thereon an electrostatic imageretaining layer, means for forming an electrostatic image on said imageretainer, means for developing the electrostatic image formed on theimage retainer to form a toner image, an oscillating electric fieldapplied between the image retainer and a developer feeding carrier todevelop said electrostatic images, means for superposing a plurality oftoner images on the image retainer, and means for transferring the tonerimages onto a recording paper in one step..]..[.2. The apparatus forreproducing multiplex images according to claim 1 wherein the surface ofa developer layer on a developer feeding carrier does not contact withthe surface of the toner image on the image retainer..]..[.3. Theapparatus for reproducing multiplex images according to claim 2comprising means for superposing at least one portion of a spotdistribution exposure of the preceding image exposure and at least oneportion of a spot distribution exposure of the following imageexposure..]..[.4. The apparatus for reproducing multiplex imagesaccording to claim 3 wherein the spots of the preceding image exposureand the following image exposure are varied in size..]..[.5. In anapparatus for reproducing multiplex images comprising an electrostaticimage formed on an image retainer, means for developing theelectrostatic image formed on the image retainer by using a developerconsisting of a plurality of components, means for superposing aplurality of toner images on the image retainer, the improvementcharacterized in that said developing means operates under a conditionmentioned below:

    0.2≦V.sub.AC /(d·f)

    [(V.sub.AC /d)-1500]/f≦1.0

where V_(AC) is an amplitude (V) of AC component of developed bias, f isa frequency (Hz), and d is a gap (mm) between the image retainer and adeveloper feeding carrier for a feeding developer..]..[.6. The apparatusfor reproducing multiplex images according to claim 5 wherein the gapbetween the image retainer and the developer feeding carrier is largerthan the thickness of the developer layer formed on the developerfeeding carrier during developing..]..[.7. The apparatus for reproducingmultiplex images according to claim 5 comprising multiplex images formedby using developers in order from the developer having a smallerabsolute value of mean charge quantity..]..[.8. The apparatus forreproducing multiplex images according to claim 3 comprising multipleximages formed by reducing successively the amplitude of the AC componentof electrical field applied between the image retainer and the developerfeeding carrier during development..]..[.9. The apparatus forreproducing multiplex images according to claim 5 comprising multipleximages formed by increasing successively the frequency of the ACcomponent of electric field applied between the image retainer and thedeveloper feeding carrier during development..]..[.10. In an apparatusfor reproducing multiplex images comprising an electrostatic imageformed on an image retainer, means for developing the electrostaticimage formed on the image retainer by using a one-component developer,and means for superposing a plurality of toner images on the imageretainer, the improvement characterized in that said developing meansoperates under the following condition:

    0.2≦V.sub.AC /(d·f)≦1.6

where V_(AC) is an amplitude (V) of AC component of developing bias, fis a frequency (Hz), and d is a gap (mm) between the image retainer anda developer feeding carrier for a feeding developer..]..[.11. Theapparatus for reproducing multiplex images according to claim 10 whereinthe gap between the image retainer and the developer feeding carrier islarger than the thickness of a developer layer formed on the developerfeeding carrier during development..]..[.12. The apparatus forreproducing multiplex images according to claim 10 comprising multipleximages formed by using developers in order from the developer having asmaller absolute value of mean charge quantity..]..[.13. The apparatusfor reproducing multiplex images according to claim 10 comprisingmultiplex images formed by reducing successively the amplitude of the ACcomponent of electric field applied between the image retainer and thedeveloper feeding carrier during development..]..[.14. The apparatus forreproducing multiplex images according to claim 10 comprising multipleximages formed by increasing successively the frequency of the ACcomponent of electric field applied between the image retainer and thedeveloper feeding carrier during development..]..[.15. The apparatus forreproducing multiplex images according to claim 10 wherein theelectrostatic image retaining member of the image retainer consists of aphotosensitive member..]..[.16. The apparatus for reproducing multipleximages according to claim 2 wherein the electrostatic image retainingmember of the image retainer consists of a dielectric member..]..[.17.An apparatus for reproducing multiplex images comprising an imageretainer having thereon an electrostatic image retaining layer, meansfor forming an electrostatic image on said image retainer, means forsuperposing a plurality of toner images on the image retainer, transfermeans for transferring the toner images on a recording paper by onestep, and means for developing the electrostatic image formed on theimage retainer to form a toner image wherein the surface of a developerlayer on a developer feeding carrier does no contact with the surface ofthe toner image on the toner retainer other than during the first timedevelopment..]..[.18. In an apparatus for reproducing multiplex imagescomprising an electrostatic image formed on an image retainer, means fordeveloping the electrostatic image formed on the image retainer by usinga developer consisting of a plurality of components, and means forsuperposing a plurality of toner images on the image retainer, theimprovement characterized in that said developing means operates under acondition mentioned below:

    0.2≦V.sub.AC /(d·f)

    [(V.sub.AC /d)-1500]/f≦1.0

where V_(AC) is an amplitude (V) of AC component of developing bias, fis a frequency (Hz), and d is a gap (mm) between the image retainer anda developer feeding carrier for feeding developer; the gap between theimage retainer and the developer feeding carrier is larger than thethickness of a developer layer formed on the developer feeding carrierduring development; multiplex images formed by reducing successively theamplitude of AC component of electric field applied between the imageretainer and the developer feeding carrier during development, andmultiplex images formed by increasing successively the frequency of ACcomponent of electric field applied between the image retainer and thedeveloper feeding carrier during development..]..[.19. In an apparatusfor reproducing multiplex images comprising an electrostatic imageformed on an image retainer, means for developing the electrostaticimage formed on the image retainer by using a one-component developer,and means for superposing a plurality of toner images on the imageretainer, the improvement characterized in that said developing meansoperates under a condition mentioned below:

    0.2≦V.sub.AC /(d·f)≦1.6

where V_(AC) is an amplitude (V) of AC component of developing bias, fis a frequency (Hz), and d is a gap (mm) between the image retainer anda developer feeding carrier for feeding developer; the gap between theimage retainer and the developer feeding carrier is larger than thethickness of a developer layer formed on the developer feeding carrierduring development; multiplex images formed by reducing successively theamplitude of AC component of electric field applied between the imageretainer and the developer feeding carrier during development; andmultiplex images formed by increasing successively the frequency of ACcomponent of electric field applied between the image retainer and thedeveloper feeding carrier during development..]..[.20. An apparatus forreproducing multiplex images comprising an image retainer having thereonan electrostatic image retaining layer, means for forming anelectrostatic image on said image retainer, means for developing theelectrostatic image formed on the image retainer to form a toner image,means for eliminating residual electric charges, means for superposing aplurality of toner images on the image retainer, and transfer means fortransferring the toner images on a recording paper by one step..]..[.21.The apparatus for reproducing multiplex images according to claim 2comprising a two-component developer having a toner and an insulatingcarrier..]..[.22. The apparatus for reproducing multiplex imagesaccording to claim 2 comprising a one-component developer consisting ofan insulating toner..]..[.23. The apparatus for reproducing multipleximages according to claim 2 wherein the image retainer consists of aphotosensitive image retainer having a photoconductive photosensitivelayer and a transparent insulating layer, in that order, laminated on anelectroconductive supporting member, and means for forming anelectrostatic image by primary charging, secondary charging, imageexposure simultaneous with the secondary charging and sequential uniformexposure, said primary charging dropping a potential of theelectrostatic images substantially to zero..]..[.24. The apparatus forreproducing multiplex images according to claim 2 wherein the imageretainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layerlaminated on an electroconductive supporting member, in that order,means for forming the electrostatic image of a first toner image byprimary charging, secondary charging and sequential image exposure, saidprimary charging dropping a potential of the electrostatic imagessubstantially to zero, and means for forming the electrostatic image ofa second toner image by primary charging, secondary charging, andsequential image exposure, said primary charging dropping a potential ofthe electrostatic images substantially to zero..]..[.25. The method ofreproducing multiplex images according to claim 2 wherein the imageretainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layerlaminated on an electroconductive supporting member, in that order,means for forming the electrostatic image of a first toner image byprimary charging, secondary charging, image exposure simultaneous withthe secondary charging and sequential uniform exposure, said primarycharging dropping a potential of the electrostatic images substantiallyto zero, and means for forming the electrostatic image of a second tonerimage by primary charging, secondary charging, image exposuresimultaneous with the secondary charging and sequential uniformexposure, said primary charging dropping a potential of theelectrostatic images substantially to zero..]..[.26. The apparatus forreproducing multiplex images according to claim 2 wherein the imageretainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layer,in that order, laminated on an electroconductive supporting member, andmeans for forming the electrostatic image of a toner image by uniformprimary charging on said photosensitive image retainer in the firstdeveloping, uniform secondary charging in a plurality reversed to saidprimary charging and sequential image exposure, said primary chargingdropping a potential of the electrostatic images substantially tozero..]..[.27. In a color image forming apparatus comprising an imageretainer, means for forming a color image on said image retainer,transfer means for transferring on a transfer member the color imageformed on said image retainer by said color image forming means, andfixing means for fixing on said transfer member the color imagetransferred on said transfer member by said transfer means, theimprovement characterized in that said color image forming meanscomprises only one latent image forming means, and a plurality ofdeveloping means for visualizing a latent image formed on said imageretainer by said latent image forming means, said one latent imageforming means being used repeatedly in one cycle of a reproduced imageforming step..]..[.28. The color image forming apparatus according toclaim 27, wherein said latent image forming means comprises means forapplying an electric charge, a light source, a deflector for deflectinga light irradiated from the light source, and a scanning optical systemarranged between said light source and said image retainer..]..[.29. Thecolor image forming apparatus according to claim 28 wherein saidelectric charge applying means comprises a primary charger and asecondary charger..]..[.30. The color image forming apparatus accordingto claim 28 wherein said light source in said scanning optical system isa laser light source..]..[.31. The color image forming apparatusaccording to claim 30 wherein said deflector in said scanning opticalsystem is a rotary polygon mirror..]..[.32. The color image formingapparatus according to claim 27, wherein said developing means isarranged in the vicinity of said image retainer in non-contactfashion..]..[.33. The color image forming apparatus according to claim27 wherein an alternating electric field is applied between said imageretainer and said developing means..]..[.34. The color image formingapparatus according to claim 27 comprising electrical charge eliminatingmeans for eliminating a residual electric charge on said image retainereach time at which said latent image forming means is used, said chargeeliminating means being arranged in the vicinity of said transfermeans..]..[.35. The apparatus for reproducing multiplex images accordingto claim 17 consisting of a single means for forming an electrostaticimage on said image retainer, and at least two of said means fordeveloping said electrostatic image formed on the image..]..Iadd.36. Anapparatus for reproducing multiplex images comprising an image retainerhaving thereon an electrostatic image retaining layer, means forcharging said image retainer, means for forming an electrostatic imageon said image retainer, means for developing the electrostatic imageformed on the image retainer to form a toner image, an oscillatingelectric field applied between the image retainer and a developerfeeding carrier to develop said electrostatic images, means forsuperposing a plurality of toner images on the image retainer, means fortransferring the toner image onto a recording paper in one step,andmeans for superposing at least one portion of a dot exposure of thepreceding image exposure and at least one portion of a dot exposure ofthe following image exposure wherein the surface of a developer layer ona developer feeding carrier does not contact the surface of the tonerimage on the image retainer..Iaddend..Iadd.37. The apparatus forreproducing multiplex images according to claim 36 wherein eachmultiplex image is developed by repeated use of said means fordeveloping in which an amplitude of an AC component of electric fieldapplied between said image retainer and a developer feeding carrier forfeeding developer is reduced with each of said repeated use of saidmeans for developing..Iaddend..Iadd.8. The apparatus of claim 36 whereinthe electrostatic image retaining member of the image retainer consistsof a dielectric member..Iaddend..Iadd.39. The apparatus of claim 36wherein said developer layer comprises a one-component developerconsisting of an insulating toner..Iaddend..Iadd.40. The apparatus forreproducing multiplex images according to claim 36 wherein the imageretainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layer,in that order, laminated on an electroconductive supporting member, andmeans for forming an electrostatic image by primary charging, secondarycharging, image exposure simultaneous with the secondary charging andsequential uniform exposure, said primary charging dropping a potentialof the electrostatic images substantially to zero. .Iadd.41. Theapparatus for reproducing multiplex images according to claim 36 whereinthe image retainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layerlaminated on an electroconductive supporting member, in that order,means for forming the electrostatic image of a first toner image byprimary charging, secondary charging and sequential image exposure, saidprimary charging dropping a potential of the electrostatic imagessubstantially to zero, and means for forming the electrostatic image ofa second toner image by primary charging, secondary charging, andsequential image exposure, said primary charging dropping a potential ofthe electrostatic image substantially to zero..Iaddend..Iadd.42. Theapparatus for reproducing multiplex images according to claim 36 whereinthe image retainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layerlaminated on an electroconductive supporting member, in that order,means for forming the electrostatic image of a first toner image byprimary charging, secondary charging, image exposure simultaneous withthe secondary charging and sequential uniform exposure, said primarycharging dropping a potential of the electrostatic images substantiallyto zero, and means for forming the electrostatic image of a second tonerimage by primary charging, secondary charging, image exposuresimultaneous with the secondary charging and sequential uniformexposure, said primary charging dropping a potential of theelectrostatic images substantially to zero..Iaddend..Iadd.43. Theapparatus for reproducing multiplex images according to claim 36 whereinthe image retainer consists of a photosensitive image retainer having aphotoconductive photosensitive layer and a transparent insulating layer,in that order, laminated on an electroconductive supporting member, andmeans for forming the electrostatic image of a toner image by uniformprimary charging on said photosensitive image retainer in the firstdeveloping, uniform secondary charging in a plurality reversed to saidprimary charging and sequential image exposure, said primary chargingdropping a potential of the electrostatic images substantially tozero..Iaddend..Iadd.44. The apparatus of claim 36 wherein said dotexposure of the preceding image exposure and said dot exposure of thefollowing image exposure are varied in size..Iaddend..Iadd.45. Theapparatus of claim 36 wherein said developer layer comprises atwo-component developer having a toner and an insulating carrier, saidinsulating carrier having a resistivity of at least 10⁸ Ω-cm when avoltage for generating an electric field of 1,000 V/cm isapplied..Iaddend..Iadd.46. An apparatus for reproducing multiplex imagescomprising an image retainer having thereon an electrostatic imageretaining layer, means for forming an electrostatic image on said imageretainer, means for superposing a plurality of toner images on the imageretainer, transfer means for transferring the toner images to arecording paper in one step, and means for developing the electrostaticimage formed on the image retainer to form a toner image wherein, in theformation of each multiplex image, the surface of a developer layer on adeveloper feeding carrier does not contact the surface of the tonerimage on the image retainer other than during the development of thefirst toner image, wherein the image retainer consists of aphotosensitive image retainer having a photoconductive photosensitivelayer and a transparent insulating layer, in that order, laminated on anelectroconductive supporting member, and said electrostatic imageforming means comprises a primary charging means, a secondary chargingmeans which exposes said image retainer to an image light simultaneouslywith the secondary charging and sequential uniform exposure means, saidprimary charging means dropping a potential of the electrostatic imagessubstantially to zero..Iaddend..Iadd.47. An apparatus for reproducingmultiplex images comprising an image retainer having thereon anelectrostatic image retaining layer, means for forming an electrostaticimage on said image retainer, means for superposing a plurality of tonerimages on the image retainer, transfer means for transferring the tonerimages to a recording paper in one step, and means for developing theelectrostatic image formed on the image retainer to form a toner imagewherein the surface of a developer layer on a developer feeding carrierdoes not contact the surface of the toner image on the image retainerother than during the first development wherein the image retainerconsists of a photosensitive image retainer having a photoconductivephotosensitive layer and a transparent insulating layer laminated on anelectroconductive supporting member, in that order, and saidelectrostatic image forming means comprises a primary charging means, asecondary charging means and sequential image exposure means, saidprimary charging means dropping a potential of the electrostatic imagessubstantially to zero..Iaddend..Iadd.48. An apparatus for reproducingmultiplex images comprising an image retainer having thereon anelectrostatic image retaining layer, means for forming an electrostaticimage on said image retainer, means for superposing a plurality of tonerimages on the image retainer, transfer means for transferring the tonerimages to a recording paper in one step, and means for developing theelectrostatic image formed on the image retainer to form a toner imagewherein the surface of a developer layer on a developer feeding carrierdoes not contact the surface of the toner image on the image retainerother than during the first development wherein the image retainerconsists of a photosensitive image retainer having a photoconductivephotosensitive layer and a transparent insulating layer laminated on anelectroconductive supporting member, in that order, means for formingthe electrostatic image of a first toner image by primary charging,secondary charging, and image exposure simultaneous with the secondarycharging and sequential uniform exposure, said primary charging droppinga potential of the electrostatic images substantially to zero, and meansfor forming the electrostatic image of a second toner image by primarycharging, secondary charging, and image exposure simultaneous with thesecondary charging and sequential uniform exposure, said primarycharging dropping a potential of the electrostatic images substantiallyto zero..Iaddend..Iadd.49. An apparatus for reproducing multiplex imagescomprising an image retainer having thereon an electrostatic imageretaining layer, means for forming an electrostatic image on said imageretainer, means for superposing a plurality of toner images on the imageretainer, transfer means for transferring the toner images onto arecording paper in one step, and means for developing the electrostaticimage formed on the image retainer to form a toner image wherein, in theformation of each multiplex image the surface of a developer layer on adeveloper feeding carrier does not contact the surface of the tonerimage on the image retainer other than during the development of thefirst toner image, wherein said developer comprises a two-componentdeveloper having a toner and an insulating carrier, said insulatingcarrier having a resistivity of at least 10⁸ Ω-cm when a voltage forgenerating an electric field of 1,000 V/cm is applied, means forgenerating an oscillating field between said image retainer and saiddeveloper feeding carrier..Iaddend.